I know "a lot" of people will use lexan in their hatches and the little rear windows on 300zx's and what not to try and reduce the weight, but as far as I knew, you can't use it for the door windows if you want it to still go up and down. It scratches really easy, and i was always told that it flexes too much to only be supported on the bottom and kind of on the sides, that it will bind up on the tracks and it will never seal tightly against the weather stripping.
The only way to use it is to glue it all the way around. So I'm told.
and as far as I know, it is only illegal to use as a windshield. everywhere else is fair game.