I only implemented the query bits, though. I’m selfish.
(Oh, OK, the only reason I didn’t do the submission part, too, is that I can’t make up my mind whether cddb.el and musicbrainz.el should share the same editing mode or not. I think perhaps.)
[Update: That felt like a cop-out, so I’ve started implementing MusicBrainz submitting. I needed some way to get a MusicBrainz-compatible CD Table-Of-Contents listing, so I hacked up cd-discid to do that.]