current_menu_parent for custom post type and custom url

Ended up hooking the nav_menu_css_class filter, and adding the appropriate class when needed.

Like this:

add_filter( 'nav_menu_css_class', 'add_parent_menu_class', 10, 2 );
function add_parent_menu_class( $classes, $item ) {

    $post = get_post();
    if ( ! $post ) {
        return $classes;
    }

    if ( $item->url === get_bloginfo( 'url' ) . '/custom_url' && get_post_type() == $this->post_type ) {
        $classes[] = 'current-menu-parent';
    }

    return $classes;
}