On http://musicbrainz.org/doc/Picard_Tag_Mapping it implies that "MusicBrainz Work Id" should be available, but testing on a release which Picard requested as http://musicbrainz.org/ws/2/release/79180d6f-cace-35c7-a305-9c7924faedef?inc=release-groups+media+recordings+puids+artist-credits+labels+isrcs+artist-rels+release-rels+url-rels+recording-rels+work-rels+recording-level-rels+work-level-rels+tags doesn't lead to these variables being available internally despite being in the response.

Needs some more investigation later.