I am using a single custom tag that contains all the MusicBrainz IDs (Album Artist, Artist, Recording, Work, etc.) in a concatenated format that gets populated when I run a song/album through Picard.
(there are quite a few types of MusicBrainz IDs, so having (sacrificing) a custom tag for each one of them is not an option.
Then to be able to make further use of them I have setup a couple of virtual tags.
(one will have the 'album artist id', another one the 'artist id', another the 'release group id', etc. etc.)
One of them is absolutely ideal and perfect to know if any track is identical to another one:
It's the MusicBrainz 'recording' ID.
But since the Duplicates Manager does not allow to use virtual tags, I can't use it to find tracks that are without any doubt duplicates.
Being able to use this virtual tag for me (but I am guessing for some other users too) would be the only 100% reliable way to find actual duplicates.