Tags like MB Track ID, MB Album ID, then stuff like (by now non-MB-only as they are supported by all major formats) Album Artist, Album Artist Sort Name (e.g. "Holiday, Billie" for Billie Holiday), Track Artist Sort Name, and other things like these, are not supported.

Supporting them should be pretty trivial, and i think is also essential in the sense that most other libraries or frameworks that can read metadata support them, and more and more players make use of them as well[citation needed] :P

OK anyway, a good overview over the specifications is here:

http://wiki.musicbrainz.org/PicardQt/TagMapping

The first column describes what the field is, and lists then in subsequent columns how exactly it must be written for all major (and not so major) formats.

I've uploaded files which are tagged to a very large extent with these tags in ogg/vorbis, mp3 and mpc format here:

http://futurepast.free.fr/STATIC/mbtags/

In particular, i'd first of all propose the inclusion to these tags, just so there's a scope on this:

"Hard" List:
- MB Track ID
- MB Artist ID
- MB Album ID
- Album Artist
- Album Artist Sort Name
- Artist Sort Name
- Album Sort Name
- MusicIP PUID (PUIDs are based on a fingerprinting system coming from a collaboration between MusicBrainz and MusicIP, which is going to replace the old , and much less reliable TRM system in approx. 3 months)
- Release Status
- Release Type
- Release Country
- ASIN
- Podcast
- Podcast URL

Somewhat arbitrary, but sensibly seeming additional list:
- Composer
- Lyricist
- Perfomer
- Remixer
- Engineer
- Producer
- Conductor
- Lyrics

Certainly, implementing the whole list would be ideal, and maybe that's even more sensible than an initial partial implementation.

In any case I'm willing to do the implementation or at least help out, I'd just need to know where to start in the gst code.