This probably is a bit late, but in a general sense Effective Altruism sounds like what you're looking for, although the main emphasis there is the "helping others as much as possible" rather than the "rationalists" part, but there's still a significant overlap in the communities. If both LW and EA are too general for you and you want something with both rationality and utilitarian altruism right in it's mission statement... I'm sure there's some blog somewhere in the ratioinalist blogosphere which is devoted to that specifically, although it might be just a single person's blog rather than a community forum.

Incidentally, if you did find- or found- a specific community along those lines I'd be interested in joining it myself.