WooCommerce - Custom sorting options (asc/desc)

gistfile1.txt

/**
 * This code should be added to functions.php of your theme
 **/
add_filter('woocommerce_get_catalog_ordering_args', 'custom_woocommerce_get_catalog_ordering_args');

function custom_woocommerce_get_catalog_ordering_args( $args ) {
	if (isset($_SESSION['orderby'])) {
		switch ($_SESSION['orderby']) :
			case 'date_asc' :
				$args['orderby'] = 'date';
				$args['order'] = 'asc';
				$args['meta_key'] = '';
			break;
			case 'price_desc' :
				$args['orderby'] = 'meta_value_num';
				$args['order'] = 'desc';
				$args['meta_key'] = '_price';
			break;
			case 'title_desc' :
				$args['orderby'] = 'title';
				$args['order'] = 'desc';
				$args['meta_key'] = '';
			break;
		endswitch;
	}
	return $args;
}

add_filter('woocommerce_catalog_orderby', 'custom_woocommerce_catalog_orderby');

function custom_woocommerce_catalog_orderby( $sortby ) {
	$sortby['title_desc'] = 'Reverse-Alphabetically';
	$sortby['price_desc'] = 'Price (highest to lowest)';
	$sortby['date_asc'] = 'Oldest to newest';
	return $sortby;
}

Can you sort by custom sort options that include meta_key, meta_value, and meta_compare. Essentially I am trying to only show products with a specific meta value such as:

case 'title_bulk' :
$args['orderby'] = 'meta_value';
$args['order'] = 'asc';
$args['meta_key'] = 'product_packaging';
$args['meta_value'] = 'Bulk';
$args['meta_compare'] = '=';
break;

replace $_SESSION with $_GET and magically, totally custom sorting starts working. found this at http://www.galalaly.me/index.php/2013/05/woocommerce-sort-by-custom-attributes/