Shellac dissolves in alcohol, so it's easy to remove. I would do that if I were doing steam cabinet rusting before the boil. When I'm just putting a solution like Pilkington's on, I don't have a problem with bore rust. I can see how coating the crown of a muzzle might get a little solution inside, though, so the shellac idea would guard against that. You could also pour some liquid asphaltum (pitch) through the chamber and bore and let it dry. It's sold by art supply places as an etch-resist for printing plates. It dissolves out with odorless mineral spirits.
The boiling itself, assuming good water, is something you dry immediately while the metal is still boiling hot and evaporates water off fast. Not only does that not cause bore rust, at that temperature it actually forms a micro-thin layer of blue oxide on the surface by reacting with oxygen in air. Not only does it not cause rust, it protects the steel. Not a lot of protection, but a little. If you have hard water, the deposits can block that blue layer forming and attract moisture later. I used either distilled water or deionized water. Both work. I don't have one, but a reverse osmosis system should produce good water for it, too. I don't know about rain water. It's acidic in some places. I'd want to check that and titrate it to neutral pH (7) if needed.
Since it takes a minimum RH of around 30% for rust to form on iron, letting the part cool in a cabinet with a small heater will also protect it. If you have a temperature of 75°F and 70% RH, you only have to raise the temperature of the same air to 102°F or warmer to get the RH below 30%. Not very warm. This is why the Goldenrod heaters work in a gun safe.