I actually have the California black plate on my car, on the front. In the rear I have a year of manufacture plate that the registry approved. Fortunately I only found one or I'd have to use both on the car.

No one (in authority) has ever questioned why I have the plate on the front, but I just tell people that it shows it was a California car (since they can't see under the hood and view the smog pump).

The only time I was questioned was about the back plate, from what appeared to be an off-duty state trooper.