Do you also store the original document? From a quick overview it seems you only store the hash. A typical situation that might occur is that a client hashes his document, a couple of years later he want to proof priority but doesn't have the original document anymore (perhaps he appended it, fixed spelling errors etc). Now his priority statement is worthless.

Why not provide a facility for the client to store his document along with the hash, encrypted off course. He can encrypt the document himself or you can provide an application that does it for him.

Charge extra for this feature...