I've got a musket with a live round loaded. Bore is heavily corroded and pitted. Nipple is broken and rusted in. Any advice on removing the ball and powder?

You need a ball screw; that is how it is done in the field. Or you can unscrew the breech plug, but a ball screw or a worm (goes on the end of the ramrod) is easier. Wet the powder first and I only say that to make you feel better. You can get ball screws from Track of the Wolf. Once you pull the ball, the powder can be washed out with water. If it was dry, you could pour it out. How do you know it is loaded and not filled with wasps nests, mud, or?
Or send me the barrel and I will pull it for you.

Take rifle to a safe place. Heat chamber with a oxy/act torch and wait. Either the round will fire or the lead will melt.

Keith

Please don't do that; you don't know what is down there; many old guns have been blown up by people putting smokeless powder in them 100 years ago, not knowing any better. In fact, my Grandfather once told me of just such a feat by him and his buddies, back in the early 1900s. They opened a shotgun shell and poured the contents down the bore of a percussion shotgun and touched it off. He said pieces were probably still falling and he told me that in 1962.

Boy, no since of humor here.

I get it, but some guys I know would think you were serious and happily blow their fingers off. I have a friend who is that gullible and would do anything re reads no matter how dumb it would look to most other people. I wish I had the money he has wasted on useless crap and stupid things. Anyway, it appears that the OP has abandoned his post and probably doesn't care any more anyway.