John, I remember that somebody on Cruise Critic was actually thinking about doing this on another cruise line ... only that, they would be leaving a guest book in the library of the ship. It would look like any other book so it would be hidden but the location of the book would be posted on CC so that other CC people that were on that cruise ship would know where to find the guest book in the library and would write down their thoughts of their cruise. Just another variation ... :)
Masaki