I dont have an undertail like that but I would put a small amount of silicone or other easily removable adhesive just to help the bracket stay to the undertail. I personally would be afraid the machine screws would not hold 100% effectively in the fiberglass. That is just MHO. If you guys say it's perfectly safe and won't back out or slowly strip out from vibration thanI just did this yesterday. I used the CF backet that came with the undertail and then made a stainless piece that to mount to that because the thing they give you is nowhere big enough to mount a US sized plate on. The holes on the CF piece were not center so put it where you want it and then drill... I use Stainless bolts with nylon nuts, they don't come loose. I added a 3/16 spacer between the plate and the bracket to allow the screw heads not to be up against the plate. I am looking today for a plate light because like you MA says the plate has to be in the middlee BLAH BLAH BLAH and have its own light or no sticker or get a fine if you get pulled over... I was thinking one on those LED plate light bolts you can get at the car crap stores. A single one mounted in front of the plate facing down should be ok. I'll let you know what I find.
just my .02