Dan,
I don't know the answer to your specific question, but even without a contract, I'm pretty sure a classy guy like Bill Coore, for example, has run it by a past client when approached to design another new course nearby. It's simply a thoughtful courtesy, to get the past client's thoughts on the situation, to start; and then to possibly decline the potential new job if in fact it could have a detrimental effect on the past client in question.
If I'm ever in such a situation, I plan to do the same.