I would like to nominate Darcs as a candidate backend for this, and also
a way to test the abstractedness of the interface (since Darcs is fairly
different).

It'd be interesting to see how to deal with merging updates.

Some hastily sketched out ideas (which are not necessarily necessary for
Darcs integration; just from the perspective of someone more experienced
with Darcs than Roundup): http://wiki.darcs.net/Ideas/BugTracker