There are a few places. There is teh Meeting Hall, which is a bit more casual - players start their own threads and keep things moving. If anything becomes too outlandish or godmodding, then a moderator could step in.
If you want to RP against NPCs run by a moderator, might be best to go for the RP training further down the RP over-forum. I believe there are moderators there that run smallish RPs for people to get used to it. I've not got any personal experience there.
Finally, there are White Scenarios. These are heavily controlled by moderators, and require a number of players to start one up. However they do give XP for completion of them.
Okay.. so the more popular one would be the meeting hall I am guessing?
EDIT: Also.. as another question.. when Role-Playing on the forums, lets say the group of people Role-Playing are on their way to land on Tatooine.. does that mean that your character location should be actually set to go to Tatooine too? Or is that something thats slightly different? :S
The Meeting hall does seem to be the most frequented.
Nothing that happens in the Meeting Hall (or the White Scenarios AFAIK), has any effect on the Combine game itself. Some Meeting Hall Threads ask that you don't bring any items, NPCs or entities that you don't also own in-game, and, of course, you can't RP a Jedi if you're not one in-game, but i've never seen restricting your in-game location as a requirement.
There is a Hail function built into the Combine. It and Darkness Messages (DMs) are set up to allow direct in-game communication.
I've not personally tested it, but I believe if two player-characters are in the same room, they can talk directly to each other, so there's opportunity for RP there. Technically speaking you could RP with the NPCs in-game, though they have very basic and limited scripts, and you probably wouldn't find it rewarding at all.
I don't think there's any direct Emotes in-game, like waves or the like.
There's also IRC #swc-ig, which I haven't tried rping in. It's supposedly a Cantina-setting.