Odds are freezing water will stand just as much chance of bursting your barrel. If it's a tight seal the grease will work.
Heating a bit may char the cloth enough to get it out without damaging the barrel...old wrought iron barrels are soldered, brazed and otherwise hot worked at higher temperatures with no I'll effects (low carbon negates the need to anneal as well).
Seating the whole mess down against the patent breech, trickling a grain or two of powder in and firing it out is yet another option. Depends what your jag is made of - if its steel or has steel fittings it may mar the bore on departure.
You could also preheat the barrel then eye-dropper some gas or ether in there then put a cap on and fire it if you wanted to get fancy... might avoid having to seat it back against the breech but I'm not sure about that... all I know if that gas in a heated vessel is one hell of a propellant.