Okay, my soon-to-be Pinnacle Club friend just sent me the following:
"The Grand Suites and above have first dibs on cabanas. They are asked if they are interested though an email from the Concierge. Now that they have to pay for them (they were originally free to suite guests) , a lot of them probably decline, leaving some cabanas available. After boarding any passenger can go to the Guest Relations Desk and ask if there are any cabanas available. I'm pretty sure the Concierge has a few held back for people with access to the Concierge Lounge but not in a suite, such as Pinnacle and Diamond Plus members."
The bottom line is, yes, you can get a cabana and not be in a suite or even be a high level Crown & Anchor member. I strongly suggest you make a bee-line for the Guest Relations desk as soon as you board and get your request on file.