I created a library to add data to Musicbrainz and a library for adding data to Discogs (not published, but I used it successfully).
I haven’t used them in a year or so, so it’s possible/likely that they need to be tweaked to work with the current versions of the websites. I’ll probably get around to that sometime this year.
Since I’m a 20th-century throwback, I wrote them in C with an eye to writing desktop and command-line apps, and wrappers for languages such as ruby/python/perl.
If you’re looking to do a web or mobile app, I’m not sure how much use they’d be for you.