Last October, we had permits and reservations for 2 backcountry campsites in the Mt. Sterling area. The second night, we changed our mind and headed for Laurel Gap Shelter, a non-AT shelter. Don't know if it was "full" via reservations but there was plenty of room.
I don't like breaking the rules but I don't think any harm was done. If the shelter had been full, we were prepared to move on.