I have been on many cruises and this remains an ongoing problem. Actually,your solution was something I thought of a few years ago. I have witnessed prime chairs sitting empty the entire day with no one even attempting to use them. They obviously had found something more attractive to do and didn't have the courtesy to go collect their cheap flip-flops until time for dinner. This is not fair and something needs to be done in a diplomatic, fair way. To save chairs for those in a group is acceptable, providing someone in the group stays with the chairs until the others arrive. If I'm traveling with others, I want to be able to sit in a deck chair and talk with my family/friends.