wpsc_google_elements

The filter wpsc_google_elements is called for each product included in the Google Merchant Centre field. Allows you to add additional elements to the feed, either for particular products, or globally across all items.

function my_custom_google_elements ($elements) {
        $custom_site_element = array( 'meta_key' => 'g:product_type',
                                      'meta_value' => 'Luggage');
        $elements['elements'][] = $custom_site_element;
        return $elements;
}
add_filter('wpsc_google_elements', my_custom_google_elements);