I'd like this too. Couple of days ago I made a similar suggestion to switch tag panels at View > Tag panel. But using the toolbar would work great too.
Another thing comes to mind. Currently Mp3tag saves the active tag panel in usrfields.ini. When you load another tag panel it takes over as the main tag panel, so when you configure it's saved to the same usrfields.ini. So you lose your previously active tag panel if you haven't made a separate backup of that one. I'd prefer if Mp3tag would save the tag panels separately to e.g. usrfields_1.ini and usrfields_2.ini so you could keep them separated and be able to switch between them without worrying about the backup.