I had a similar request a while ago:
My suggestion was to use buttons on the top toolbar to quickly switch between what tags are displayed.
Currently there's a priority like this: APEv2 > ID3v2 > ID3v1
So you have to disable reading ID3v2 in the options and refresh the file list in order to see the ID3v1 tags. That's very cumbersome and takes a lot of time when you're editing hundreds of MP3s.
I think Mp3Tag should read all tags at once (hold them in memory) and make the different kings accessible via buttons (applying to normal placeholders) and via special placeholders/functions for a fixed and permanent view in the file list view.
And a simple point/select/copy&paste from one tag to another should also be great. Because when I want to write to a specific tag I also have to go to the options and adjust the "Read" settings... that's killing the workflow. Same for deleting a specific tag... 