Form/Style would be a new set of complexities, as this is another classification on its own.
Thoughts will surely differ on this, but when narrowing this down to using a system for genres, Form/Style in my mind is a somewhat different entity:
While genre and sub-genres would have more strict relationships, and should try to adhere to some pseudo-academic correctness, Form/Style/Keyword/whatever would be a less restricted listing of all sorts of words and terminologies that a user may want to have available to apply to his liking.
This will allow to use the fuzzy human brain capacity for handling chaos. Which is probably an essential factor to make this all usable in practice.
For example for genres, such a 'chaos' list could contain words related to music and genres such as: independent, alternative, ballroom, crossover, elevator music, industrial, serenade, soft rock, Top 40, etc.
I would like to be able to apply them independent from a selected genre.(I have such a list, I'll see if I can include it in my updated genre list, probably later tonight)
This 'chaos list' could of course also be an unrelated independent tag, but it could be nice to have it included in an xml sheet such as you are proposing.
When using such a system for instruments, I am sure you could also come up with a list of words that do not fit in the musical instruments taxonomy from an academic standpoint, but you would still like to have available there?