I'm at MOC now, and asked about cabanas for the first time...except in really high-demand weeks, it sounds like a fair number of the made-in-advance reservations get cancelled once people get to the property and their plans change, so even if you don't reserve far in advance you may be able to get one. This week, there appear to be plenty open.
That said, as long as you don't want bar/food service to come to you (and don't mind the sand

), the beach umbrella/2 lounge chairs for $25 per day isn't a bad second choice.