The best way to solve it then would be to merge pve - pvp and let the roll players transfer if they want. But your point about f2p and trolling, same thing will happen on their server, people will roll on their server just to troll the RPers.
I think it's less of an issue then them having no one to play with because their server dies.
My only issue with them having the choice?
You could split the RP community in two thus with dribs and drabs coming onto the RP server then, so that in itself may cause it's own doom.
I think it would have to be an all or nothing option. And a heavily moderated /rpchannel.
I have absolutely no experience in a RP server but an issue that springs to mind is say for eg in a WZ. You have a premade of 4 RP'ers and a premade of 4 PVP'ers. I'm a bit of a gambling man, but eventually you'll have a situation where say the RP'ers talked in /ops with some form of RP, and the PVP'ers are like, ROFL *** IS THIS LET'S MAKE FUN OF THIS... People get ignored and that WZ goes to absolute horrible, then incites a *****torm in /1 on the fleet afterwoods.
I know this comes down to trolls being trolls, but I could see things like this happening regularly.
It'd be interesting to hear some RP'ers weigh on this debate imho.