Nice idea but with all related things the ultimate question remains whether courts will actually recognize it.

Currently courts will still consider paper-signed and scanned PDFs as legally binding, so any verification on top of that is superfluous to them.

More realistically, you take an oauth when you take the stand at the court, and if a signed document was altered by the counterparty you'd say so truthfully, if it weren't, you'd say so truthfully, and the penalty of that oauth purjury is high enough that most people wouldn't do it. Cryptography not needed.