If you can show me a list of Vorbis comments (the tagging system that FLAC uses) beyond the minimal one, then I'd be happy to point out which one to use.

This is really rhetorical, because I'm 99% sure no such list exists. The Vorbis folks may have a good tagging system, but it's of no use unless someone defines standard tags for them. Especially since developers develop to provided specifications. I'm sure that no mp3 playback software would support gain controls if the ID3v2 folks hadn't specified a tag for it. As such, even if you can find somewhere someone who's defined such a Vorbis comment name, no playback software will support it, as it's not even approaching a standard.