Ahhh, yes. My mistake. I missed the part where you siad "similar to how we are able to do with custom tags."
So the answer is no, it can't be done.
My question to you is why would you want to do this? Understand that MB is a Music -Manager- and Player, with the emphasis being on 'manager.' In order to manage the music files, tags need to be save to where they would be expected as specified by ID3.
So what would one or two of your scenarios be where this would be useful to you. And I assume no one else.