*cast Revive*
So I ended up buying some of the Nikwax wash-in stuff a little while ago, and I got the chance to test out my newly waterproofed windbreaker in a light rain today. It worked pretty well. I could see water beading up on my sleeves and I stayed pretty dry for the whole commute (about half an hour). I can't say how it'll fare in a heavier shower, but so far I'm definitely impressed.