Along with responses you're likely to get, I would suggest also making contact with some Phnom Penh boys on Gay Romeo. Even if you're not going to meet them, if anyone would know the gay scene there, they would. If you find a boy who is responsive, you might even consider making him an offer to personally show you around the gay scene. Chances are good he would not only know the places where farang go, but he might also be able to show you lesser known places too (which would interest me much more than touristy spots).

I've never been to Phnom Penh, but if I were to go that's what I would do.