Not at all, I was just checking MBID of the 6th symphony on the MB website to show that the search can be done, not that it would be user-friendly.
I don’t think I ever tried to change tags like this, I usually just tag with picard and update MPD (I use mostly my directories structure to browse anyway).
No idea if e.g. filling the comment tag from another tag can be done with picard or beets, I would probably try to write a loop for lltag.
It might be easier just to try to recompile a recent MPD if you already have your files tagged correctly rather than retagging them all.