I also can't imagine you're not allowed to run prototypes at sanctioned events. For stock racing I see the point of this, but for modified?
There have been numerous IFMAR events where prototypes were raced, and not just pre-production cars, like the infamous Associated Stealth car for example.
Anyways, I'm just curious to see if I can beat my own lap time with a car I built myself compared to an off the shelf car. And enjoy the design and building process of course!