The work around that I'm using is similar to your master record - template idea.
I created dummy mp3 files, one for each value that I wanted to show up in the drop down box and then made the files read only so the values wouldn't change.
So now when I want to tag some actual files, I select the dummy files along with the ones I want to update and all the values I need show up in the drop down box. I'll do anything to avoid typing. 
It would be nice if I could provide the values in an external file that Mp3tag would read in at start up. Or better yet, an internal table that the user could maintain. Perhaps a future enhancement?