If you are on the 'meet and great' thread for your sailing you may want to post there that your looking for one. If someone in your group is going to cancel they may let you know when they will relinquish it and you can be waiting to grab it.
I have never wanted a cabana but on our b2b last October there was one open for each week so I booked them. I talked to dh and decided it was just a waste for us so I put out in my groups that I was going to release them and someone in the group was able to snag them. Its worth a try, you never know.