What if you brought the water line into the pond beneath the freeze line, then took a float valve setup like what Esshup posted to, and separated the float from the valve? This would allow you to place the valve itself underwater on the feed line, away from the danger of freezing. You would need to fabricate a new linkage system to connect the float, which will still be on the surface, down to the valve.
If you're able to keep the area of the float ice-free with aeration, I think it could work. In regards to the debounce that JKB speaks of, I keep drawing a comparison to an item in my line of work....hydrostatically driven machines often utilize dampeners, to prevent jerkiness while under operation. You've seen those little "shocks" that hold up hatchbacks and trunk lids? They manufacture those in some pretty small sizes.
I believe one of those could be adapted to reduce unwanted motion of the float.
