I guess I would do it the way we do in dogs. Owner of the sire gets pick of the clutch, period- and that's it. Unless you work out a deal, for 2nd pick or something before hand, that's how it's done in the dog world.
If someone loans/leases a female dog, that person acts as the dog's breeder/owner and pays all breeding expenses including stud fees, etc, and the person who loaned out the dog gets whatever was agreed upon- usually a fee, or maybe pick of the litter.
I have never heard of such a situation with snakes. The risk of infection or disease in my collection would concern me with snakes. Dogs are vaccinated against devestating disease, but snakes are not.