I hadn't looked at RAR until your email just now prompted me to do so.
RAR does seem like a sensible extension to the existing oAuth protocol. I'm not sure, however, that it overlaps much (in terms of intention, at least) with UMA: the core oAuth assumption that the client is acting on behalf of the resource owner, and that the resource owner and the requesting party are the same individual, remains true.
It might, however, be the case that UMA flows could benefit from the additional informational in a RAR-type request when deciding (for example) whether to issue an RPT.
--&e