I have some stuff in my shed which they use on aircraft cockpits, if I recall its silicone based and water just beads on the cockpits and runs off. I used to use it on my car windscreen and providing I was doing at least 30mph I did not have to use my wipers unless the rain was very heavy. Brilliant stuff it was as my windscreen was always clear, especially the areas where the wipers never touched.
I have been meaning to try it out on my cycling glasses but never got round to it yet. Perhaps my speed might be too slow, but it might be offset by the fact my glasses are square onto the airflow. When I get chance I'll give it a try and let you know.
Just went into my shed, the stuffs called "Repcon" rain repellant, tried it on my glasses, sprayed them with water, lightly blew on them and all the beadlets of water dispersed off the edge off the glasses.
For once now, I want a rainy day for a proper test