After some more thought on the subject.
The main state museum at Harrisbug, PA has a fairly large collection of Pennsylvania longrifles, pouches and powder horns. They rotate items in the display cases but ask they not be photographed. They do allow a few people to examine examples stored away but you almost need a big politician to pull strings to get to them.
Another possibility. You might consider calling Dixon Muzzleloading. Chuck Dixon is semi-retired now. Sometimes in the shop and sometimes not. His son Greg and his wife Brenda now run the shop. Chuck did have a good size collection of locally produced longrifles along with some pouches and horns. You might try talking to them.
Mad Monk