Since I also live in a state that does not require front end plates, I had required that my Bu not have the front plate installed. When my car arrived (from out of state), it had the front plate on it. The salesman remembered me letting him know I did not want a car with a front plate. They were then going to fill the holes with silocon and paint over it, but I said I was not interested in a car with holes painted over. So they swapped my grille section with that of another of the same color -- and I got waht I wanted.
That doesn't help answer your question, but I think the other recommendations about just placing a design plate on the front is the best idea -- rather than trying to fill it and paint over it.
