If I were sailing alone, I'd probably not be sailing with the jib. this may solve the issue there.

I would also be pleasure sailing, not racing, so I could get by with a little extra equipment on board to keep me attached or in proximity of the boat.

I would figure a "kill-switch" system would do one or several of the following:
- stop the boat
- turn the boat upwind (stopping progress)
- flip the boat (stopping progress)

As for a tether, I think it might work well (as long as there is a way to detach from the tether if you're getting dragged to death). I figured if I were washed over with the mainsail cleated on a reach, it might take a long time for the boat to round up and stop. Releasing the main via the snap shackle on the block might help achieve that faster?

Perhaps in addition to this tether which pops the mainsheet, it would remain connected to the boat on one end and a sea-drogue at the other end? That way I could either remain attached to the tether (and therefore the boat) or deploy the drogue (or both) to attempt to slow the boat's progress and direction?