The best location for the caribeaner would likely be one that if dragged, would keep your face somewhere near the surface (think being dragged by parachute on your back), but that would likely get all tangled up if you were using the mainsheet.

How about a "kill switch" concept? You could run a line from your harness to a snap shackle on the lower mainsheet block. Fall off and the snap shackle opens, releasing the mainsheet entirely.

This would at least slow/stop the boat and could be used in concert with something to keep you somehow attached to the boat...