Today I made a version of rvw! for submitting music reviews to Google Base.
Unfortunately it fails as submissions require a valid session token that you can only get when adding reviews via Google's interface. Shame, really, it could have been useful.
Update: It works now - you need to have Greasemonkey (I'm using v0.6.3 and Firefox 1.5 RC2) and this little helper script installed.
You need to be signed in to Google Base before posting as well, otherwise it gives an error.