Is this part of the new ID3 xml-based tagging system? I seem to remember that it would let you store arbitrary data in the ID3 tags now. If so (and you can code any), go look on the nullsoft website for info on the new tagging system. I can't remember whether or not they release code libraries.... but you should at least be able to find some useful related libraries, since the new all-magical, all-the-time winamp uses the new tagging system.