I've spent the past weeks also working on adding support for custom tag sources to Mp3tag for Mac. I've initially thought that adding a completely new framework for developing and supporting custom tag sources would be a good idea. — However, over the past months I've learned that many users asking for custom tag sources know them from the Windows version and expect it to work in a at least similar fashion.
So I took the plunge and revisited the really ancient code and re-implemented the Web Sources Framework for Mp3tag for Mac.
I'm now at a stage where I soon want to invite interested users for beta testing. If this is you, please use Help → Send Feedback and send me an email to request beta access. Please also include in your email which Tag Sources you're most likely to use.