I called Sandals - we are going to be in St. Lucia for te day in December. They were very helpful, and told me that the day "passes" are up to the individual managers, and depend on the degree to which they are booked up. They gave me the name and phone number for the manager of the location we wanted, and recommended that we contact him directly a few days before we arrive to try to arrange to check out the facilities for a day. If they are not close to full up, I was told we have a good shot.
Hope that gives you some ideas!