Never had a cop stop while pulling me over to take a good, hard look at the windshield that I had in the mustang I used to own. It was Lexan, and illegal as hell. But with proper installation and a nice AS1 strip of tint at the top, it didn't look any different under cursory examination. Now, knowing this, I wouldn't hesitate to use lexan on a rig, being as I have never had a cop bother to even look at the rear windows, much less try and determine the material of their construction.
All in all, it's your ass if you get caught, but I don't think your chances are high for it.