We owned a non-Marriott HHI property some years ago and had a late July week. It was always crowded and at times pretty darn hot, but not without its merits. (Not in high hurricane season, lots of interesting people etc.etc.) The traffic was the big thing and the constant pre-planning to go out to eat, just like any other beach vacation area). We have since unloaded that property and have successfully traded our MGV as well as a Cape Code property into HHI (Surf Watch, Barony, Monarch and one of the other ones in Sea Pines) Labor Day week for the past 4 years. If July is your only shot, there are certainly plenty of properties to pick from. Just get your request in ASAP.
There used to be local owners using the properties even though they were not actually booked in, but I don't know if that is the case. Some of the more experienced Tuggers can address that.
I would guess that any previous history with trading/exchanging etc. into any given property will be altered somewhat in the upcoming year (s), given the current state of economy. Since some Marriotts have already posted their annual maint. fee, you may be surprised at how quick you can lock in July 09. Good luck!