In case Steven is open to doing this, here is a table that should make it easier:
Internal Name | ID3v2 | Vorbis | APEv2 | iTunes MP4 | ASF/Windows Media |
musicbrainz_artistid | TXXX:MusicBrainz Artist Id | MUSICBRAINZ_ARTISTID | MUSICBRAINZ_ARTISTID | ----:com.apple.iTunes:MusicBrainz Artist Id | MusicBrainz/Artist Id |
musicbrainz_discid | TXXX:MusicBrainz Disc Id | MUSICBRAINZ_DISCID | MUSICBRAINZ_DISCID | ----:com.apple.iTunes:MusicBrainz Disc Id | MusicBrainz/Disc Id |
musicbrainz_originalartistid | TXXX:MusicBrainz Original Artist Id | MUSICBRAINZ_ORIGINALARTISTID | n/a | ----:com.apple.iTunes:MusicBrainz Original Artist Id | MusicBrainz/Original Artist Id |
musicbrainz_originalalbumid | TXXX:MusicBrainz Original Album Id | MUSICBRAINZ_ORIGINALALBUMID | n/a | ----:com.apple.iTunes:MusicBrainz Original Album Id | MusicBrainz/Original Album Id |
musicbrainz_recordingid | UFID:http://musicbrainz.org | MUSICBRAINZ_TRACKID | MUSICBRAINZ_TRACKID | ----:com.apple.iTunes:MusicBrainz Track Id | MusicBrainz/Track Id |
musicbrainz_albumartistid | TXXX:MusicBrainz Album Artist Id | MUSICBRAINZ_ALBUMARTISTID | MUSICBRAINZ_ALBUMARTISTID | ----:com.apple.iTunes:MusicBrainz Album Artist Id | MusicBrainz/Album Artist Id |
musicbrainz_releasegroupid | TXXX:MusicBrainz Release Group Id | MUSICBRAINZ_RELEASEGROUPID | MUSICBRAINZ_RELEASEGROUPID | ----:com.apple.iTunes:MusicBrainz Release Group Id | MusicBrainz/Release Group Id |
musicbrainz_albumid | TXXX:MusicBrainz Album Id | MUSICBRAINZ_ALBUMID | MUSICBRAINZ_ALBUMID | ----:com.apple.iTunes:MusicBrainz Album Id | MusicBrainz/Album Id |
musicbrainz_trackid | TXXX:MusicBrainz Release Track Id | MUSICBRAINZ_RELEASETRACKID | MUSICBRAINZ_RELEASETRACKID | ----:com.apple.iTunes:MusicBrainz Release Track Id | MusicBrainz/Release Track Id |
musicbrainz_workid | TXXX:MusicBrainz Work Id | MUSICBRAINZ_WORKID | MUSICBRAINZ_WORKID | ----:com.apple.iTunes:MusicBrainz Work Id | MusicBrainz/Work Id |