WP Query for custom post-type categories...

OK, I'm ALMOST THERE with an addon and I narrowed it down to, I'm pretty sure, one bugger line.

In essence, I need to find a replacement for this ($wp_query->query_vars['category___in']) that will access the categories of the custom post type. Once done presto-bango, I'm pretty certain the rest of this will work as intended.

Any thoughts??