Fogged lenses would clear but in really cold temps, the fogs freezes and becomes frost. Frost doesn't clear unless you wipe it away (warming it up is impossible while riding). I've had the same problem skiing in really cold temps where I've had to have two layers covering my mouth and nose. On the bike, if I cover my nose at all, my glasses fog. I only get a little fog at the corners if I leave my nose exposed and cover my mouth. This works good enough for me. I rode a quarter mile without glasses on when it was 5F and my eyes were extremely dried out in that short period. I've had luck with dish detergent at 15F (I was able to cover my nose) but it failed miserably at 10 degrees colder.

I'm thinking the best solution would be a scarf or neck gaiter that could be easily moved up and down to cover your mouth and nose on downhills or flat areas where you are moving quickly but not working too hard and then expose your mouth and nose while climbing to reduce fogging. This is eaiser said than done though because when it's cold enough to need this sort of setup, I'm wearing big mittens which leave me just enough dexterity to brake and shift. Our winters aren't long or consistent enough to give me enough practice either.