I'm quite certain that any sort of waterproofing spray would kill all the breathability of the jacket. You'd be better off getting a cheap plastic jacket like this: http://www.nashbar.com/bikes//Produc...2_170065_-1___

and then use your wind jacket for it's intended purpose.

The problem with riding in the rain is that you're going to get wet no matter what.... whether it's soaking through the jacket from the outside, creeping in through the openings, or it's your own sweat precipitating on the inside. If you simply must ride in the rain and must stay dry, the best course of action is to wear rubber rain gear, and not ride fast enough to sweat. Otherwise, make sure you're warm enough, and learn to love the wet.... and never ever stop or you'll freeze to death.