I also inadvertently tested the mainsheet tether, and don't recommend it.
Maiden voyage on my old Tornado, pre 2 trap days, blowing like stink. Hiking strap tie down broke. On my way past the mainsheet blocks while exiting on the leeward side I uncleated the main. Thought I was pretty smart about it, bobbed up and took a deep breath to yell at my crew who was happy out on the trap and unaware of my unplanned exit. Before I could shout, my legs were yanked out from under me. As luck would have it, the mainsheet that was wrapped around both my legs was sheeted in tight by my drag and the boat took off like a banshee. I don't know if you've ever tried to do a sit up while getting dragged through the water backwards, literally leaping out of the water like a crazed dolphin, but it ain't easy. Somehow I managed to get hold of the line and pull myself back to the boat. Until I was right there at the crossbar shouting at him, my crew had no idea of the show he missed. 30 years later I still have rope burn scars on my legs.

Maybe a tether attached to one end of the rear cross beam instead of the middle would do the trick. 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.