I think the two tiered system would work, provided the QC folks would stay on top of things and respond promptly.

Shouldn't be a problem really, get a good group of 5-10 people to start with and things should go fairly quickly as far as the QC process is concerned.  Since your not reviewing all the models on the site, only one or possibly two submissions when an artist requests access to the second tier, this will keep the workload down considerably.

Then all you'd have to do is just police the site a bit and respond to any reports of possible scammers on the second tier, once a scammer is caught they loose any right to any revenue the might have made from all those phony clicks anyway.

Once word gets around the scammers will look elsewhere for an easier target, and all that will be left are the honest model makers.