Not sure if the event needs to be on the AT, but if not my suggestion would be World's End State Park
The timing of the Fall MAHHA would put usually be at the peak of the fall foliage color in this area. There are plenty of trails for hiking including the Loyalsock Trail (59 miles), The Loyalsock - Link Loop (22 mile loop), and ~ 20 miles of trail inside the park. There are some nice canyon vistas, and Loyalsock creek as well. I think they have sites that can accommodate large groups.