Even if you have an engineers certificate, they can still defect you for putting a non-standard part on your car.. The only difference is that you can get it cleared right away. This is the absolute worst case scenario if the police officer wants to be a real jerk.
Moral of this story, if you put something that's not standard on your car, that is a defect and that is illegal.
My friend got defected for have Recaros in his DC2R.. Even though they came out standard.
Having ADR approved seats doesn't make it legal to be used on the street, however, it will be all good for CAMS approval to be used on the track if scrutineers are picky.

