>>[...] assuming you could never truly trust the client to not mess with/read the data.
We have to wait for fully homomorphic encryption[1] to realize this, or a clever implementation of zero-knowledge proofs. Your proof of concept may work now if you can implement partial homomorphic encryption, perhaps something similar to CryptDB[2] or ZeroDB[3]. I read more about this in a recent Zdnet article that gives a brief summary[4].
This is the conclusion that I came to (without knowing there was a word for homomorphic encryption, thanks!). I really think this is the (ideal) future of monetizing the internet.