It's interesting that you say that ("rather specialized") because it baffles me that neither Mp3tag nor Picard (by default) offer what I believe to be the most important artist tag; that is a multivalue (or delimited) albumartist tag.
I'm relatively new to the collaboration tags (Artist A feat Artist B), but how is a server/player supposed to make use of the musicbrainz_albumartistid tag without a matching 1-1 albumartist tag?
If we take Watch the Throne as an example:
MUSICBRAINZ_ALBUMARTISTID: f82bcf78-5b69-4622-a5ef-73800768d9ac/164f0d73-1234-4e2c-8743-d77bf2191051
ALBUMARTIST: JAY Z & Kanye West
Are all consumers of that data supposed to split ALBUMARTIST on all the possible join phrases until the count of artists is equal to the count of mbid? '&' is quite common in band names so that seems quite a risky thing to do.
Enabling 'Show all available non-standard MusicBrainz metadata tags?' delivers the following:
MUSICBRAINZ_ALBUMID: e08a3b6c-22ff-423a-8706-adbd45203698
MUSICBRAINZ_ALBUMARTISTID: f82bcf78-5b69-4622-a5ef-73800768d9ac/164f0d73-1234-4e2c-8743-d77bf2191051
ALBUMARTIST: JAY Z & Kanye West
MB_RELEASEGROUP_ARTISTCREDIT_ARTIST_NAME: JAY‐Z/Ye
MB_RELEASEGROUP_ARTISTCREDIT_NAME: Jay-Z/Kanye West
So we have the standardised name (ARTISTCREDIT_ARTIST_NAME) and the release group variant (ARTISTCREDIT_NAME) but still not the release.
I appreciate that there's an ARTISTS tag, but how would one go about applying a sort value?
MUSICBRAINZ_ALBUMID: 70be723e-7a51-4ffd-81f0-10f1d01c4890
TRACK: 8/17
ARTISTS: Miley Cyrus/WATT/Elton John/Yo‐Yo Ma/Robert Trujillo/Chad Smith
ARTISTSORT: Cyrus, Miley feat. Watt, Andrew, John, Elton, Ma, Yo‐Yo, Trujillo, Robert & Smith, Chad
I've come from a UPnP server background where multivalue albumartist(sort) and artist(sort) tags are required, so this is all a little foreign to me.

