I'm still waiting on an answer about the availability of beach chairs, but as for the cabanas, the answer is yes, they can be reserved in advance (and I would advise doing so, as a lot were already reserved a month out from my stay). If you go to the "spa" section of the hotel website, there is a link halfway down the page to view cabana availability by date, pool area, cabana type, and pricing.
Ill be there in less than 2 weeks and will share my experience with the beach chairs.