check this out, best accessory i've ever seen for the Pacific NW!
http://www.wakeworld.com/MB/Discus/messages/3183/203350.html i've often thought about just having a screen like you describe, though. i believe you could just have a zipper installed across the front of the bimini, and snaps along the top of the windshield. then it's just a matter of getting the clear sheet made to fit.
i already have a tonneau cover for the bow, and a simple screen like that would make a big difference in cold & rain.