Having PVP'd outside the arena, I think the system works very well as is - the person in question pm'd, verified, and set up a lovely bit of pretentious dialogue for Andrew to poke fun at before Andrew died a swift and heinous death. The worst part was waiting for my tormentor to get "properly" dressed! (don't ask).I agree though that the fluid nature of our reasons for being on; just gathering, only on for a minute to talk to someone, looking for rp, heading to a quest...make that initial tell of "hey, I'd like to pvp" pretty critical. nothing stopping you from making agreements outside that though, but I'd hesitate to put my name on a list because I'm not sure what I am doing from day to day or how much time I have.
I still have that screenshot of hostiling SeHky, Keela and you and how you all failed to kill me at level 7 hehe.. don't brag too much.
From personal experience, I know that people send plenty of tells in the midst of RP about generally unrelated matters, so a quick pair of tells asking for and agreeing to PvP shouldn't really be much of a burden.