Hi--
We've done that at Hilton Head! And yes, you can get a cash reservation for less than the posted rates, but they have to be available. As I understand it, if someone has traded out of the system, there will be cash reservations available through member services. They are typically less than what you see posted.
If that doesn't work, I wouldn't rule out going on line and looking at Expedia or somebody for reservations. I've seen Hilton Head out there. Sometimes if they have too many cash reservations available I think they put them on the services to increase their chances of filling the place. Once they do so they no longer book them.