Workaround to use Custom Taxonomies in WordPress SEO titles

<?php
/*
 * Workaround to use Custom Taxonomies in WordPress SEO titles
 *
 * @author: brasofilo
 * @licence: GPL
 *
 * Drop this code in your theme's functions.php
 *
 * Instructions: adjust the function **brsfl_wpseo_taxonomy_title** with your custom values
 *
 * Usage: go to the page /wp-admin/admin.php?page=wpseo_titles
 *        and set the tag of your CPT to %%cf__my_cat_title%%
 *        ATTENTION to the double underscore after %%cf
 *
 * How it works: the Custom Field "_my_cat_title" is automatically populated with the current categories of the desired CPT
 *               and the value of the field will be used for you custom SEO tag
 *               ATTENTION: an underscore at the beginning of the Custom Field prevents it from showing up in the post editing screen
 *
 */

add_action ('save_post','brsfl_wpseo_taxonomy_title',10,2);

/*
 * @desc Makes an automatic Custom Field with the current categories of a Custom Post Type
 *
 * ADJUST THIS 3 VALUES IN THE FUNCTION TO MATCH YOUR NEEDS
 * @movie Name of the Custom Post Type
 * @category Name of the Custom Taxonomy
 * @_my_cat_title Name of the Custom Field
 *
 */
function brsfl_wpseo_taxonomy_title($post_ID, $post) {
    $type = get_post_type($post_ID);
    if ( 'movie' != $type ) return;
    $the_cats = brsfl_wpseo_get_terms($post_ID, 'category');
    update_post_meta($post_ID, '_my_cat_title', $the_cats);
}

/*
 * @desc Grabs the categories associated with a post
 *
 * @returns Comma separated list of categories
 *
 */
function brsfl_wpseo_get_terms($ID, $taxonomy) {
    $terms = get_the_terms( $ID, $taxonomy);
    $return = array();
    foreach($terms as $t) {
        $return[] = $t->name;
    }
    return join( ", ", $return );
}