Is there a way to substitute the "desc" version of description field with the "©des" version instead, or maybe output it to both?
file types: mp4 m4a m4v
It's unfortunately not possible with Mp3tag. Which app is relying on ©des?
QuickTime - Wouldn't say "Relying on it". I just wanted the Field to show up when i go to "Show Movie Inspecter" in QuickTime
Mistake, mp4 is actually using “dscp” and not “©des” — why all this? 
what a mess
There are many ways to tame these differences down. I would suggest making edits in QuickTime for a test file. Make sure you include this “suspect” description field. Save it and close QT. Then open the same file in mp3tag and check the Extended Tags window to see what field shows the same data. You should then be able to use mp3tag to populate that same field in other files that should then appear in QT.
I have many m4v and mp4 files in my library that show correct metadata and have all been updated with mp3tag.