Dynamic template serving, change theme_root using add_filter from current theme

Some plugins use the 'template', 'option_template' and 'option_stylesheet' to dynamically serve (alternative) wordpress templates. For example, Nathan Rice’s ServeDefaultToIESix.
For Example –

add_filter('template', 'change_theme');
add_filter('option_template', 'change_theme');
add_filter('option_stylesheet', 'change_theme');

function change_theme() 
{
    // Alternate theme
    return 'AwesomeTheme';
}

Above code only works from a wordpress plugin. What i need, is to switch to alternative template, located in one of the subfolders of the current theme(template). Examples: display an alternative HTML5 theme, serve mobile users a minimal version of the site.. etc.

I tried to use ‘theme_root’ and ‘theme_root_uri’ as below. But that isn’t working.

// Extra lines to change the theme's root.
add_filter('theme_root', 'change_theme_root');
add_filter('theme_root', 'change_theme_root_uri');
//
add_filter('template', 'change_theme');
add_filter('option_template', 'change_theme');
add_filter('option_stylesheet', 'change_theme');

function change_theme() 
{
    // Display Alternate theme
    return 'AwesomeTheme';
}

function change_theme_root()
{
    // Return the new theme root
    return WP_CONTENT_DIR . 'themes/OrigTheme/lib/AltThemes';
}

function change_theme_root_uri()
{ 
     // Return the new theme root uri
     return get_bloginfo('wpurl') . '/wp-content/themes/OrigTheme/lib/AltThemes';
}

Is this the correct way of doing it? Or does anyone know of the possible way to do so?
Thanks in advance.

You could also write your own simple get_template_part alias function:
The following allows 3 subfolders for template parts that sit in a theme root folder named devices.

<?php
// STYLESHEETS
    function print_device_styles( $client="desktop" ) 
    {
        $client = apply_filters( 'set_theme_client', $client );
        wp_enqueue_style( $client.'-css' );
    }
    add_action( 'wp_head', 'print_device_styles', 11 );

// TEMPLATE PARTS
    function get_device_template_part( $args = array( 'file' => NULL, 'suffix' => 'default', 'client' => 'desktop', 'media' => 'screen' ) 
    {
        if ( ! $args['file'] )
            wp_die( sprintf( __('You have to specify a file name if you want to load a template part with the %1$s function.', 'textdomain', '<pre>get_device_template_part()</pre>' );

        $template_path = user_trailingslashit( get_stylesheet_directory().'/devices/templates-'.$args['client'] );
        $ui_path = user_trailingslashit( get_stylesheet_directory().'/ui/css-'.$args['client'] );
        $ui_suffix = '.css'; // could be switched between '.dev.css' & '.css'

        // add styles & template directory
        if ( is_condition_mobile() ) 
        {
            $args['client'] = 'mobile';
            $args['screen'] = 'handheld';
        }
        elseif ( is_condition_tablet() )
        {
            $args['client'] = 'tablet';
            $args['screen'] = 'handheld';
        }

        // register styles
        // wp_register_style( 'mobile-css', /theme_root/ui/css-mobile/mobile.css, false 'handheld' );
        wp_register_style( $args['client'].'-css', $ui_path.$args['client'].$ui_suffix, false, $args['screen'] );

            // Requires PHP 5.3+ (for lower versions, use a plain function).
        add_filter( 'set_theme_client', function('set_theme_client') { return $args['client'];} );

            // {$template}-{$suffix}.php
        if ( file_exists( $template_path.$args['file'].'-'.$args['suffix'].'.php' ) )
        {
            require( $template_path.$args['file'].'-'.$args['suffix'].'php' );
        }
            // {$template}.php
        elseif ( file_exists( $template_path.$args['file'].'.php' ) )
        {
            require( $template_path.$args['file'].'.php' );
        }
            // {$template}-default.php
        elseif ( file_exists( $template_path.$args['file'].'-default.php' ) )
        {
            require( $template_path.$args['file'].'-default.php' );
        }
    }

// CALL THEM in a template file
// This will require a file named {$template}-{$suffix}.php from your devices folder
// based on your conditional functions that detect your devices
// If not found, it will search for a file named {$template}.php
// and if it wasn't found it will search for a file named {$template}-default.php
    get_device_template_part( array( 'file' => 'nav', 'suffix' => 'main' ) );
?>

Feel free to add your conditional device detection to: https://gist.github.com/886501