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;
}