I had a similar idea. I did a different knot though. I did basically the sliding part of a scaffold knot with a separate piece of rope (looks like two wraps around the main rope). Without a loop, this left a tag end on both sides, with one towards the quick link. I left that tag end a little long and then gave it a few wraps of gorilla duct tape. When I slide it down against the quick link, the quick link goes between the tether rope and the tag end of the slider that is wrapped with duct tape to be made stiff. It has worked really well all last season. Periodically, I grab both tag ends of the sliding scaffold knot piece and pull to tighten the whole assembly up. The scaffold I'm discussing would basically be a double fisherman's knot I think, since it isn't forming the loop.
Your knot is smaller, more symmetrical and clean looking, however. I used the the same rope as the tether (predator).