Another idea.

My wife just 'lanolized' a wool diaper cover for our youngest. "What's that?" I asked.

Apparently, lanolin in the stuff secreted from sheep skin that makes wool as resilient in the elements as it is. In processing into textiles, the lanolin is removed from the fiber. Treating it again with lanolin restores it to the way it was supposed to work. I'm not sure I really understand, because they say wool is simultaneously water-repellent and absorbent. Maybe it allows a sheep to be wet, but comfortable. I'd be ok with that.

So, I'll buy a pair of wool trousers from the thrift store, lanolize them, and then wear them out in the rain. Will report back with findings....