This seems like it's up @hiccup's alley.
I think you'll probably have to start by making a Custom tag for the subgenre tag, though.
Indeed MusicBee has no out-of-the-box 'subgenre' tag, so you will have to create a custom tag for it.
Doing a forum search should result in some pointers on how other users have put this to use.
Maybe helpful: If you are using the 'Tag Hierarchy Explorer' and the (my) 'Enhanced Genre hierarchy list' ( https://getmusicbee.com/forum/index.php?topic=35266.0
), it has a feature where it makes suggestions for subgenres relevant to existing 'main' genres and making it possible to handle them for filtering and displaying purposes.