How will you differentiate from and compete against the horde of other proofing services (not to mention the horde of proofing services that specialize in CVs)?
And to answer your question, most (ones I came across) of proofing services are done by software and the result is not good enough to have a confidence to send my CV to apply for job, my idea would use human to do so. This is why I think having some sort of reward will good so people can commit to it.
I'd encourage you to do some more competitive research as there are a LOT of proofing services that use humans. One can also hire proofers off of sites like Odesk, etc.
So with the knowledge that there are lots of competitors with humans doing the work, you might need to find a different strategic differentiator.
Doing more competitive research would a good path to start with and I have to say your advices/thoughts are really appreciated.