We never came across this scenario. I think if we were ever made aware by a client of the clause we would have declined the booking. In general, I think it's pretty unusual for the client to even read the contract, never mind make the bands aware of any relevant clause. If the client doesn't make the band aware of the clause, what then? I guess to cover yourself, it would have to be in the contract between band and client.
All in in all I'm glad I don't do weddings any more!