problem is when you change from steel to FRP or even aluminium (GTR guards etc) it changes the "crash worthyness" of the car, hence making it un-roadworthy.
It sucks because front guards are not structural, but in the even of a prang fibreglass/carbon splinters will spray everywhere, even more so if you hit a pedestrian.
If you have the budget to crash test all your panels (like blits did) i.e crash your car into a wall a few times then you can produce adr approved stuff.
I'd be dubious if an engineer will touch stuff like that, but it may be possible given enough cash.
In saying that I had a Carbon bonnet on the R33 for years an never got picked up on it.