I don't know about small windows- your car has to pass other safety laws and regulations.
As for HUDs I think you can have them if they can only show driver/rider related stuff while the vehicle is in operation, if the screen can be general purpose like Google Glass then it's illegal:
27602. (a) A person shall not drive a motor vehicle if a television receiver, a video monitor, or a television or video screen, or any other similar means of visually displaying a television broadcast or video signal that produces entertainment or business applications, is operating and is located in the motor vehicle at a point forward of the back of the driverâ(TM)s seat, or is operating and the monitor, screen, or display is visible to the driver while driving the motor vehicle.
(b) Subdivision (a) does not apply to the following equipment when installed in a vehicle:
(1) A vehicle information display.
(2) A global positioning display.
(3) A mapping display.
The law doesn't seem that crazy as laws go considering how crazy drivers can get ;).