I think people are reading way too much into this, I think when it came to the exam, the obvious skill and determination that Rock possessed easily overshadowed the small technicality that he couldn't pull off a bushin.

I think they would have recognised his obvious potential and just let him slip through, I doubt any strings were pulled by Gai or anyone else. Or then again, maybe he was able to barely pull of a bushin once, maybe he got lucky.