I think Neall Designs has already been here - I'll post a picture when the camera's charged. I made a pertex top for fell running with slim mesh side panels which velcro closed if required. The neck has a neoprene seal so water doesn't drip down your neck (man, I hate that

) As wind-proofness was the real function rather than water-proofness, the front zip just has a single overlap of reflective material for running back on the roads. The fit is very slim as I also hate loads of flapping material as it's pretty windy on the tops so anything which can flap does.
It would be pretty easy to take the design and make it up in waterproof stuff. I must admit that I've never been troubled with puddles on the midriff when out on the 'bents in the rain. I use an old Gore Bike Wear (really old - like 15+ years) rain smock with only a short front zip. It's really basic, elasticated cuffs, draw-cord hem and a nice baffle behind the short zip. No pockets at all. But it works fine.