I suppose I'll comment, since this came up because of a post I made.
I would gladly take the risk of loosing twice the amount of experience I normally would, plus my PvP points, plus a certain amount of gems, even whatever gold I'm carrying, if it means I can get back at someone personally.
I've sent a message in an attempt to RP, I've watched and haven't seen him rank up even once, and I've sent someone else a message in a request for assistance. Outside of doing that, setting a bounty that may not ever be collected, and hoping to find that assassin in the forest (which I've found twice since I started playing), there's nothing else I can do.
Besides pay someone each time they kill the person, which I would also do. As I said before, I'd find a way to take care of it, even if I had to go after the individuals clanmates as well.
But I do like this idea, and would definitely use it. Even with the suggested risk of experience loss.