By the way, ship island never closes. Maybe the fort, snack shop, etc. are seasonal and closed to tourists a certain time of the year, as well as the ferry that takes you out there. The rangers are always out there, just maybe not as plentiful on the off seasons. You can goto the Gulf islands national seashore website to read their rules and regs. Maybe it is possible to write someone up high within one of their commissions/committee and get a permit to do such. I dont know if that will even work, but i have been considering giving it a shot.
Also, islands constantly change their formation over time due to wind, rain, surf erosion. The north side of ship has eroded almost completely, and on the south side it has built up. The fort used to be on the south side now it is at the tideline of the north side.
The island is still several hundred yards wide where the fort, (ft. massachusetts) rests..
Check out regulation 8

GINS NPS SITE
http://www.nps.gov/guis/index.htm
Gulf Islands National Seashore
Laws & Policies
Emergencies: Park visitors should contact the park dispatch at 850-916-3010 or 911. Because of hurricane damages visitors are urged to use great care when visiting Gulf Islands National Seashore.
1. Glass containers are not allowed on the barrier islands. All litter must be removed from the Seashore.
2. Visitors should obey the park speed limits.
3. Fireworks are not allowed within the boundaries.
4. As of February 22, 2010, a new federal law allows people who can legally possess firearms under applicable federal, state, and local laws, to legally possess firearms in Gulf Islands National Seashore.
It is the responsibility of visitors to understand and comply with all applicable state, local, and federal firearms laws before entering this park. Federal law also prohibits firearms in certain facilities in the park. "Federal Facilities" are defined as a "building or part thereof owned or leased by the Federal Government, where Federal employees are regularly present for the purpose of performing their official duties." this includes but is not limited to administrative buildings, visitor centers, and within walled forts including any open grounds within.
References:
Federal Statue 18 USC section 930
Florida state statute section 790.25, & Fl. section 790.001(17)
Mississippi state statute section 97-37-1
The laws of the state of Mississippi , however, generally prohibit the possession of firearms within the Mississippi units of this park, except as allowed under specific National Park Service regulations. For those regulations, you may read Section 2.4 of Title 36 of the Code of Federal Regulations, or call or email this park.
5. Pets are not allowed on the Florida beaches, the Mississippi wilderness islands on Horn or Petit Bois or within the swimbeach at West Ship Island.
6. The use of Personal Watercraft (PWC) is based on location.
7. Fishing limits are based on state laws.
8. Metal detectors are not allowed and all historic objects and buildings are protected.
9. Plants and animals are protected within the Seashore.
10. Fires are allowed only in designated areas.
12. Drink responsibly and follow all state laws on alcohol consumption.
13. Observe closures. To help protect seagrass beds Spanish Cove and the unnamed cove near Langley and Red Fish Points on the northshore of Perdido Key are closed to combustion engines.
14. COMPENDIUM of designations, closures, permit requirements and other restrictions imposed under the discretionary authority by the Superintendent of Gulf Islands National Seashore.
15. Title 36 Code of Federal Regulations