Yeah you totally could, and before writing this plugin it's what I was doing, haha. As there is a human involved, it is very error prone though, and boy is it tedious!
A big part of DbC is that contracts are part of the public interface, not the implementation. This becomes especially important once you mesh them with Simula-style OOP, as derived classes need a way for overridden methods to widen or narrow the inherited contract.
You make this sound way easier than I would expect it to be in the general case.