Based upon all of the unknown variables with this gun, there's not too many options other than to bring it to a gunsmith who has the necessary tools to remove the lodged ball. The best reply that I did find was the one where grease being pumped into the barrel through the nipple was the best, and....I am sure that the cleanup afterwards was a tedious job. I DEFINITELY do not recommend putting heat or dumping some 4FG into the nipple and firing off the gun, unless that gun was tied to a tree and I was pulling the trigger with a 30 foot piece of string.
If you value this gun, and from what you have written you do, spend the cash and bring it to a gunsmith who will have the necessary tools and know-how/experience to remove the ball or have the necessary tools to make a took to remove the ball.