Extend Individual Menu on Page Function to individual Submenue on Page Function?

the solution is now included in a plug in!

    <?php
/**
 * Plugin Name: CE WP-Menu per Page
 * Plugin URI: http://ppfeufer.de/wordpress-plugin/ce-wp-menu-per-page/
 * Description: This plugin allows you to select a menu on a per page basis.
 * Version: 1.2
 * Author: Codeenterprise (H.-Peter Pfeufer)
 * Author URI: http://codeenterprise.de
 * Text Domain: ce-wp-menu-per-page
 * Domain Path: /l10n
 */


        /**
         * Constructor (old style)
         *
         * @uses __construct
         */
        function Ce_Wp_Menu_Per_Page() {
            self::__construct();
        } // END function Ce_Wp_Menu_Per_Page()

        /**
         * Constructor
         */
        function __construct() {
            // Backend
            if(is_admin()) {
                add_action('admin_init', array(
                    $this,
                    '_plugin_init'
                ));

                add_action('add_meta_boxes', array(
                    $this,
                    '_add_meta_box'
                ));

                add_action('save_post', array(
                    $this,
                    '_save_page_menu'
                ));
            } // END if(is_admin())

            // Frontend
            if(!is_admin()) {
                add_filter('wp_nav_menu_args', array(
                    $this,
                    '_menu_per_page'
                ));
            } // END if(!is_admin())
        } // END function __construct()

        function _plugin_init() {
            /**
             * Sprachdatei wählen
             */
            if(function_exists('load_plugin_textdomain')) {
                load_plugin_textdomain($this->textdomain, false, dirname(plugin_basename( __FILE__ )) . '/l10n/');
            } // END if(function_exists('load_plugin_textdomain'))
        } // END function _plugin_init()

        /**
         * Metabox am System anmelden
         */
        function _add_meta_box() {
            add_meta_box($this->metaboxID, __('Select the menu for this page', $this->textdomain), array(
                $this,
                'the_meta_box'
            ), $this->posttype, 'normal', 'high');
        } // END function _add_meta_box()

        /**
         * Metabox erstellen
         */
        function the_meta_box() {
            global $post;

            // Menüs abholen
            $menues = wp_get_nav_menus();
            $menu_locations = get_registered_nav_menus();

            if(!empty($menues) && count($menues) != 0) {
                // Use nonce for verification
                wp_nonce_field(plugin_basename( __FILE__ ), $this->noncename);

                // Ist bereits ein Menü gewählt?
                $menuslug = get_post_meta($post->ID, $this->metaname_menu, true);
                $menuposition = get_post_meta($post->ID, $this->metaname_menu_position, true);

                // Setting the defaultmenu
                if(empty($menuslug)) {
                    $menuslug = $this->defaultmenu;
                } // END if(empty($menu_name))

                $array_DefaultMenu['wp-default'] = new stdClass();
                $array_DefaultMenu['wp-default']->name = __('Wordpress Default', $this->textdomain);
                $array_DefaultMenu['wp-default']->slug = 'wp-default';

                $menues = array_merge($array_DefaultMenu, (array) $menues);

                echo sprintf('<p>%1$s</p>', __('Please select the menu which should be displayed on this page.', $this->textdomain));
                echo '<select name="' . $this->menu_selectname . '">';

                foreach($menues as $menu) {
                    if(!empty($menuslug)) {
                        $selected = '';

                        if($menuslug == $menu->slug) {
                            $selected = ' selected="selected"';
                        } // END if($menu_name == $menu->slug)
                    } // END if(!empty($menu_name))

                    echo '<option value="' . $menu->slug . '"' . $selected . '>' . $menu->name . '</option>';
                } // END foreach($menues as $menu)

                echo '</select>';

                if(is_array($menu_locations) && count($menu_locations) != 0) {
                    echo sprintf('<p>%1$s</p>', __('Please select the menu location, where your menu should appear', $this->textdomain));
                    echo '<select name="' . $this->position_selectname . '">';

                    foreach($menu_locations as $slug => $position) {
                        if(!empty($menuposition)) {
                            $selected = '';

                            if($menuposition == $slug) {
                                $selected = ' selected="selected"';
                            } // END if($menu_name == $menu->slug)
                        } // END if(!empty($menu_name))

                        echo '<option value="' . $slug . '"' . $selected . '>' . $position . '</option>';
                    } // END foreach($menues as $menu)

                    echo '</select>';
                } // END if(is_array($menu_locations) && count($menu_locations) != 0)
            } // END if(!empty($menues) && count($menues) != 0)
        } // END function the_meta_box()

        /**
         * Daten speichern
         *
         * @param int $post_id
         */
        function _save_page_menu($post_id) {
            // Erst mal schauen wir, ob der Nutzer das überhaupt darf
            if(!current_user_can($this->userright, $post_id)) {
                return;
            }

            // Dann prüfen wir die Nonces
            if(!isset($_REQUEST[$this->noncename]) || !wp_verify_nonce($_REQUEST[$this->noncename], plugin_basename(__FILE__))) {
                return;
            } // END if(!isset($_REQUEST['vokabel_page_menu']) || !wp_verify_nonce($_REQUEST['vokabel_page_menu'], plugin_basename(__FILE__)))

            // und nun wird der ganze Hokuspokus gespeichert
            $post_id = $_REQUEST['post_ID'];

            /**
             * Saving the Settings
             */
            if($_REQUEST[$this->menu_selectname] == $this->defaultmenu) {
                // Get the old slug
                $menuslug = get_post_meta($post->ID, $this->metaname_menu, true);

                if(empty($menuslug)) {
                    delete_post_meta($post_id, $this->metaname_menu);
                    delete_post_meta($post_id, $this->metaname_menu_position);
                } // END if(empty($menu_name))
            } else {
                // Metainformationen hinzufügen oder aktualisieren
                add_post_meta($post_id, $this->metaname_menu, $_REQUEST[$this->menu_selectname], true) or update_post_meta($post_id, $this->metaname_menu, $_REQUEST[$this->menu_selectname]);
                add_post_meta($post_id, $this->metaname_menu_position, $_REQUEST[$this->position_selectname], true) or update_post_meta($post_id, $this->metaname_menu_position, $_REQUEST[$this->position_selectname]);
            } // END // END if($_REQUEST[$this->menu_selectname] == $this->defaultmenu)
        } // END function _save_page_menu($post_id)

        /**
         * Menü im Frontend anzeigen
         *
         * @param array $args
         * @return Ambigous <mixed, string, multitype:, boolean, array, string>
         */
        function _menu_per_page($args = '') {
            global $post;

            if(!empty($post)) {
                $menuslug = get_post_meta($post->ID, $this->metaname_menu, true);
                $menuposition = get_post_meta($post->ID, $this->metaname_menu_position, true);

                if(empty($menuposition)) {
                    $menuposition = $this->defaultmenu_position;
                } // END if(empty($menuposition))

                if(is_page() && $args['theme_location'] == $menuposition) {
                    if(!empty($menuslug) && is_nav_menu($menuslug)) {
                        $args['menu'] = $menuslug;
                    } // END if(!empty($menu_name) && is_nav_menu($menu_name))
                } // END if(is_page())
            } // END if(!empty($post))

            return $args;
        } // END function _menu_per_page($args = '')
    } // END class Ce_Wp_Menu_Per_Page

    // Klasse starten
    new Ce_Wp_Menu_Per_Page();
} // END if(!class_exists('Ce_Wp_Menu_Per_Page'))

If this is about specifying your $theme_location for wp_nav_menu() , than it should be enough to check if it is the right location. And only than modify the $args of the _menu_per_page() function, which is used as callback for the wp_nav_menu_args filter. So the function looks like this:

    function _menu_per_page( $args ) {
        if(is_page()) {
            // the selected menu applies to the following location
            if ( $args['theme_location'] == 'sub-header-menu' ) {
                global $post;

                $menuslug = get_post_meta($post->ID, $this->metaname, true);

                if(!empty($menuslug) && is_nav_menu($menuslug)) {
                    $args['menu'] = $menuslug;
                } // END if(!empty($menu_name) && is_nav_menu($menu_name))
            } // END if ( $args['theme_location'] == 'sub-header-menu' )
        } // END if(is_page())

        return $args;
    } // END function _menu_per_page($args = '')