technically air bolt guns like a Seneca are loaded from the muzzle. And if one tried could you not load any air gun through the muzzle? At lease those that shoot bullets or balls and not pellets (you'd probably screw up the skirt if you tried that.)
You could pin the breech closed
I am actually working on a new barrel design right now as a matter of fact that is a drop in smooth barrel for the .45 Texan. My plan is to be able to load it breech side with paper cartridge shot shells. For muzzle loading season I will pin the barrel with a push dowel pin so it can't be loaded from the breech. I will muzzle load it with a ram rod with a .40 cal round ball and wad. Bryan from Lethal Air worked on it for me last week and it should be here any day. Stay tuned!