While this doesn't directly answer your question, it might be relevant:
My experience is that my thighs get colder in the weather you've described than my calves. That experience includes the memorable first 60 miles of my first 300k brevet, with a hard, driving rain at 35-40 degrees before the rain stopped. I survived and thrived with wool socks wrapped in plastic grocery bags, polypro tights, and good clothing on my torso and head. My riding companion was better equipped with
rainlegs. Ergo, I think you're barking up the wrong tree looking for waterproof lower legs -- the rain's going to hit your upper legs and run right down the inside of your tights.
But perhaps rain pants or gaiters will suffice for you.