Great idea, making the guy go to a nude beach. Perhaps key holders could have a way to let each other know their guys are locked. A medallion on a short, non-removable chain. When one of the ladies recognizes it, she gains instant control over the chastity slave.
I think a keyholder who wants her victim's status to be known to other women probably enjoys it more if the other women find out by seeing it directly - hence the nude beach visit.