in ws/1 you could retrieve tags for a release
http://www.musicbrainz.org/ws/1/release/dacb200a-999c-44fc-ad10-9651dbd51fa8?type=xml&inc=tags

They have now been moved to releasegroup level, but it should still be possible to retrieve them with
http://test.musicbrainz.org/ws/2/release/abb2d8b4-e6d9-3619-812b-d2582554e24f?inc=release-groups+tags

But the tags inc option is not liked, you have to do
http://test.musicbrainz.org/ws/2/release/abb2d8b4-e6d9-3619-812b-d2582554e24f?inc=release-groups
and then another query
http://test.musicbrainz.org/ws/2/release-group/3f274cce-1df4-34ce-908a-fa0785f1f90e?inc=tags
which is very inefficient.