How many bikes would there have been this year in your proposed class, just for curiosities sake?
According to the Round 6 Results, there were 8 different riders in the Aprillia Cup this season, typically 4 riders in each round. There were 2 Ninja 250's out that I saw, myself and Wade Dickson. Other than that, there were no other small bikes out that I was aware of (i.e. Hyosung 250, CBR125, maybe some 125/250 motards?). If I had been allowed to run I certainly would have, and that would have been 1 more rider than there was (though, I never did ask if I could).
I've also noticed that the CMRA has some HP-limited classes. Perhaps that's an idea to look into as opposed to 100% spec races, unless it's like the SV Cup where the manufacturer offers contingency and such.