On a recent trip to Denali, I did my usual which included using an "all weather" pack from Low-pro and my trusty umbrella. I have thought about the various products others have mentioned, here.
Then, one of the folks in our group brought out a brilliant self-made item she used. She had a "dry-sack" like you'd pack gear in for a canoe trip. Then, she cut out a small hole and created a draw string around that. Pulling the draw string tight around her lens hood, she could put her arms through the normally open end of the sack and use the camera. The only limitation was needing to push the sack up a bit to sneak her eye through to the viewfinder. Worked a lot like a Think Tank (a company I love) Hydrophobia at a lot less of a price.