Or a tether to a snap shackle that would release the entire mainsheet system from the sail but still keep it and you connected to the traveler car would be even better.

I was thinking of the same tether for a single hander... Use one of the wichard snap shackles for life harnesses that will release under load. Attach your tether (red line) to the boom with a break-away line that will keep it from inadvertently releasing the mainsheet. Make it point forward so that whichever side you go off, the green line will break and spin the shackle that direction and release. Might need a bit more complex tie at the mainsheet... first to the release then a loop down to the top of the main blocks so it will release then hold you to the boat via the blocks.