At Harbour Point a couple weeks ago the picture board advertising the Sales Presentations was a picture of the home on Hilton Head, and the front desk staff thought it was a great idea for us to go by and see it (which we didn't because we were running around doing a lot of other things.) They mentioned only the one on the island.
This is the thread from when the first home was added to the DC Explorer Collection:
New vacation option [HHI Home for DC Points Use] A search on the Beaufort County Register of Deeds doesn't have any other similar "LEASE" arrangements between MORI and HHI homeowners/property managers.
If you sign in to owners.marriottvacationclub.com and click on the "Discover Your New Options!" link in the bottom left "Vacation Homes" box, it takes you to a page with two tabs. One links to the single "HHI Private Home" and the other to the "Villas of Distinction" site with 99 more homes/villas but none of those are on HHI.
It's entirely possible that they're adding more every day either through a similar lease arrangement with the owners or through the VoD portal, but it's curious that right now there's nothing official to suggest that there are more private home options in South Carolina than the one. It might be worth noting that the website was updated immediately when the one was announced.