I reached out to Shore Beach Service, which is a private company that has supported the HHI beaches for over 30-years. I explained the need for people that want to launch anything from dinghies to catamarans from a public beach area (remember that some plantations may have access points of their own). Here is the response from Shore Beach Service for reference (Note: I will go to Burkes Beach and take some pictures and provide feedback on just how accessible it is for a catamaran, expect an update within two-weeks):
Burkes Beach is probably the easiest, though the path is a bit long. Coligny is fine as well, put they need to move it down the beach away from the crowds to launch it. Just so you know, we can tow boats out to the beach for $25. If it is a relatively short distance the $25 fee can include towing it back off as well.
Thank you for your inquiry.
Shore Beach Service
116 Arrow Road
Hilton Head Island, SC 29928
Mike P. again: If you are there for a week and communicate with shore beach service they would probably be fine with you leaving your boat on the beach in a deisgnated area. Designated means above the high tide mark but not in the dunes / sea oats.