You're obsessing over a "nothing". Many of us up in this corner of the country ride literally hundreds of times in the rain per year if we commute to work. None of us worry about a bit of water getting into the wheels. If we did we'd never have time to ride. Similarly mountian bikers run the bikes through streams and boggy areas and then just hose the bike off when they get back home, give it a shake to bounce off the worst of the water, oil the chain and then hang the things on a hook until next time.

EDIT- 2 or 3 oz of water? You riding with deep V rims? I have to admit that I never thought of the rain issue with those sort of wheels. The funny thing is that any time I've pulled a tire to change it after washing the wheels the smaller box section rims never have more than a film of water in them. Sounds like deep V rims need some scupper holes in the box section....