As you already know it was requested in the past, but it is high risk for instability. MM5 is designed to support such feature but lets make existing features stable before we add new.

musicbrainz_recordingid       Recording MusicBrainz Identifier
musicbrainz_trackid           Release Track MusicBrainz Identifier
musicbrainz_artistid          Track Artist's MusicBrainz Identifier
musicbrainz_albumid 		    Release MusicBrainz Identifier
musicbrainz_albumartistid 	 Release Artist's MusicBrainz Identifier
musicbrainz_discid 			  Disc ID if album added using CD Lookup
musicbrainz_releasegroupid
musicbrainz_workid
acoustid_id
acoustid_fingerprint

In MM5 I'm still missing all the additional id3tags from MusicBrainz, iTunes, Windows Mediaplayer etc.

Could you please provide a way to see (and even better manipulate) them?
Actually, I have to use 3rd-party Software to see the tags like
[code]musicbrainz_recordingid Recording MusicBrainz Identifier
musicbrainz_trackid Release Track MusicBrainz Identifier
musicbrainz_artistid Track Artist's MusicBrainz Identifier
musicbrainz_albumid Release MusicBrainz Identifier
musicbrainz_albumartistid Release Artist's MusicBrainz Identifier
musicbrainz_discid Disc ID if album added using CD Lookup
musicbrainz_releasegroupid
musicbrainz_workid
acoustid_id
acoustid_fingerprint[/code]and many more...

Maybe a [url=http://wiki.hydrogenaud.io/index.php?title=Tag_Mapping]mapping table like this[/url] or [url=http://help.mp3tag.de/main_tags.html]this[/url] or [url=https://picard.musicbrainz.org/docs/mappings/]this[/url] can help you to identify the missing tags?