Have you considered integrating with taxonomy? it would be very usefull and powerfull if all the metadata tags automatically create new vocabularies. these vocabularies should then be freetagging and the data retrieved from the file (trough id3 library) would automaticale populate them.
that way we could make use of all the taxonomy advantages.
i came up with this when trying to create a tagcloud with tagadelic, basing it on the genre, but this was not posible because tagadelic depends on vocabularies