I picked up two inline muzzle-loaders for $40. They were both put away without cleaning and stored in a barn. I got one apart and freed up, but the other has a frozen bolt.

The cleaning rod indicates that the frozen one is probably loaded. Is there a trick to getting the bolt loose? I can't access the breech plug because the bolt is closed, and it doesn't appear that there are any threads in the bolt itself. I'm afraid to use heat since it may be loaded, but I've considered going in the yard and trying to fire it in a safe direction.

All suggestions appreciated.

Get a bullet puller.  Are you able to pull the action out of the stock to soak in penetrating oil?
https://www.amazon.com/Traditions-Performance-Firearms-Muzzleloader-Bullet/dp/B0029M2GRM

Dispose of gun and get a new one.  Gun was probably used with black powder substitutes which are more corrosive than acid.  Suspect present gun is junk.

A piece of 2x4 to tap on the bolt handle. Spray a bunch of WD40 down the barrel and into the bolt area and let it soak for a week. Then use the 2x4 to tap/pound the bolt handle to get the bolt to turn. Don’t beat the shit out of the bolt handle.

Ballistol cleans Pyrodex very well for me. It’s rather liquid and ought to penetrate well.

Although, if it’s that bad, you probably want to hit it with 50/50 ATF and Acetone. That seems to be the best stuff these days.

Do you have a bullet puller screw thingy for your cleaning/ram-rod? Worth a shot to pull the bullet. Might get lucky and it be soft lead. After pulling a bullet you could give it a little heat. No more than like 200*F. Might want to use a heat gun with most solvents/cleaners!