I agree that Blackwater has the best options. For a group hang, it would probably be best to utilize a location accessible by car, as not all local hammockers are backpackers. It would also be good to have a site near a river or stream, as none of the sites outside of the RV camps have potable water.
The problem with the Blackwater recognized front country sites (not RV), is that there are no reservations; all first come, first served. I do recall hearing about a site on the Blackwater River on one of the bluffs. Another possibility is the Red Rock campsite. I will speak to my friend when I can about these sites and try to get some ideas.
“Indian builds small fire and stays warm, white man builds big fire and stays warm collecting firewood”—unknown
“The cure for anything is salt water - sweat, tears, or the sea”—Karen Blixen