Tags for Categories Grouping

I’m looking for a way to group tags beneath categories, as a kind of hierarchy of tags.

If I have a music site with categories based on genres – Pop, Rock, Jazz, Hip-Hop, etc., I’d like to have a list of pre-defined tags for writers to pick from based on which category it’s in – so in “Hip-Hop” there may be tags for old-school, gangsta, east-coast, west-coast, etc., but in the Jazz category I’d only want to see tags like improvisation, bebop, funk, acid, swing, instrumental, vocal, etc.

I know that tags in WordPress are “flat” but I’m having a hard time finding a plugin that groups tags based on category (there are some that group them by other things, but then I can’t find one that shows those groups in the posting page in the wp-admin).

Any ideas?