The bug in question has been fixed already so this would be more of an extension of that fix. Because of the bug mentioned in the above thread, I have a lot of guest artists located under the tag <performer:guest artist> instead of <guest artist>. The only way to fix this would be to edit the performer field manually for every track with MB. Entering "Multiple Artist Splitter" and just adding and removing a space to the performer field will, upon saving, move the tag value from <performer> to <guest artist>.
However, when you add new music files that's been tagged with i.e Musicbrainz Picard, guest artists will once again end up under <performer> because Picard tags guest artists as <performer:guest artist>. And MB only remedies this if the tags are edited/created with MB and not other tag editors.
What I'm asking for is that MB is made to recognize the tag "Performer:guest artist" and put the value in the tag <guest artist>.