I broke my windshield a couple of years ago and while I was looking for a source I considered using a lexan replacement if I came up short. In the end I did find a new glass windshield.
Beyond the reasons not to use one on a street car, I never really got a straight answer from the DMV, BAR, or the Highway Patrol on this. I made at least two calls to the BAR and DMV in Sacramento and asked a couple of CHP and local police officers. Everyone had a different answer from why not to what is lexan? It will really come to the highway patrol sign off when you get your Vin tag put on.
If the CHP guy doesn't reject it then you will get by. At least that would be my guess. Most people will assume that the windshield is glass and be looking for cracks if they inspect it at all. If the first CPH office rejects it I would just go to another one and try it there. Once the title/registration process is done there really won't be another official reason to have a look at your car for the rest of it's life in Calif.
All in all I would try it and if you can't get it signed off then you would have to sorce a glass windshield.