no, its not illegal, just zip tie ur back plate onto the metal bumper.
As long as u have a metal bumper on there its legal.
Like look at trucks, they just use a metal bar and its their bumper.
Passenger cars use bumper covers cuz metal bumpers on em would be ugly.

I drove right past 2 cops who sit and radar everyone in the school zone outside of my high school, every day for a week, and they never did anything and they obviously saw me cuz an integra without a front or rear bumper is pretty hard to miss.
I had my turn signals zip tied on the front and my front plate zip tied on, and I had my rear plate zip tied on with the lights on it also, so it was all street legal still.