I've been writing and using a C library for submitting data to musicbrainz.
Currently it supports adding and editing (many, but not all attributes of) artists, events and venues (i.e. "places").
It doesn't, however, yet support adding/editing release information -- I've never (wait-- checking... once) added release information to musicbrainz by any method.
My development has tapered off in the last few months, but I'm still successfully using it frequently.
I figure now's a good time to publish/announce the latest version of the code.
It requires libcurl and jansson libraries, which I believe are available on a wide variety of OSes.
I've written/tested/used it only on Linux, but I've tried not to use any non-portable stuff.
I haven't bothered with all the licensing/open-source decisions yet, so I'm just calling it Public Domain for now.
You can download it from http://br1an.fastmail.fm/musicbrainz/musicbrainz-write-0.2.tar.gz . There's some documentation inside the tarball.