Wow... that looks cold... :-) What about having some sort of forward/aft preventer on you? basically, have a line forward with a small block and tackle and one aft with a block and tackle. have them to a central snap shackle with a quick release - just incase. The thought is you can get settled on the wing, and then clip into the preventers, then tension the forward and the aft to keep you from lurching forward or backwards.
On another note, I'm surprised on the clip you use for your mainsheet. I would think it does not have sufficient strength. I have had those blow up on different applications.