Just a suggestion considering that 40k fans tend to be open to it. Perhaps you could do with an RP forum section? In my experience it works well if played like the Deathwatch RPG with the GM creating a scenario and a squad or team of posters each post their response. This is followed by the GM posting the next part of the story to which each participant replies their response. EXAMPLE:
GM: once upon a time there were Orks
Post1: I killed an ork once and now i shall again.
post2: I wish it were Eldar instead.
post3: OOOPs forgot my gun.
Gm: 3 forgot his weapon and died, 1 was prepared for the attack, and 2 doesnt seem enthusiastic
You get the point even though that was a poorly executed example. Just a suggestion. RP can be quite entertaining if you know how to do it correctly.
Well back in ye olde days of this forum being active, we did have a Role Playing kind of thread. Can't remember what happened, but it was a bit of fun. But now we have Deathwatch and the assorted supplements, who knows?
Perhaps when the board is fairly active, I can look into a chat room based section for genuine games of Deathwatch. Thinking out loud here, mind.