The Concierge teams holds back two cabanas for onboard booking. It used to be only one but it’s been two ever since those cabanas came back online after the hurricane a few years ago.
Onboard, the cabanas are the responsibility of the Concierge team. Therefore, they will inquire if a Concierge party wishes to have one of the two held back, or any available if there are any extra on both sides of the island. Once they have gone through all of them, they turn to the waitlist maintained by Guest services that covers regular guests.
Since you‘l be sailing Concierge, your best bet to have one if you are unable to book one before sailing is to get on the ship ASAP. You’ll be greeted by a Concierge host if you participate in the pre-boarding. Mention to that person your wish to have a cabana. They’ll get you one if they can.