Since the release group type isn't shown, people unexpectedly do things like add compilations to release groups for singles.
Setting it to NGS+1 for now, as my iterations are filling up fast with internal server errors. Feel free to remind me in a couple of weeks about this issue when the more pressing issues have been resolved.
Decided to fix within 3 months by scheduling game and final decision in http://chatlogs.musicbrainz.org/musicbrainz-devel/2011/2011-12/2011-12-22.html#T22-20-32-630380
I've submitted a quick solution to code review at http://codereview.musicbrainz.org/r/1689 (screenshot there). Available for test on http://test.musicbrainz.org