The music stores I’ve dealt with accept ISRCs as unique IDs. Unfortunately, ISRC is a centrally regulated system that is geared towards the existing music industry (the US ISRC contact appears to be an RIAA employee). I don’t know how hard it would be to set up a free ISRC assignment service for independent artists (dyndns for music) but that seems like a more commercially adopted solution than MusicBrainz IDs.