There is no doubt that diagrams can be used in contracts and frequently are. Though a diagram would not stand alone, it would constitute strong or even conclusive evidence of the meaning of the prose part of a written contract (or the meaning of words exchanged by parties in the case of a verbal contract).
Bottom line: a meeting of the minds always has reference to some context and all of that context can be relevant to determining meaning - if a diagram helps show the context in which parties are agreeing to something, it can and should be used to add clarity to a contract.
I wonder how that would work in court?
I suspect getting it to fly would be the biggest challenge, though, since our legal system is very much bound up in tradition and precedents.
Maybe if Lessig got together with a few linguists and information architects...
(define ([Person]X is-thief of: [Property]P belonging-to: [Person]Y)
(and
(X appropriates P)
(X != Y)
(X intends
((Time>now) implies not (Y has P)))
(dishonestly)))
(define dishonestly
"this is purposely left undefined in the law")
(define ([Person]X appropriates [Property]P)
"X does some action causing him to have control over P")
Too bad about that Gödel guy, though.