I heated the chuck several times on the stove (not to a bronze color) but to the point where it wasn't touchable and you can hear the moisture on your fingers sizzling. No luck.Try a little ceramic heater close to the part as I have done this on parts that do not move and it worked. The metal will expand, but the gunk will not. While the part is very warm try to move it, and once it moves then clean the parts that are hard to get to.
I pushed it apart with the press, using aluminum blocks supporting the hood and a brass compression fitting sleeve going around the jaws bearing on the body. Took around 500 pounds on the press, but that's just a guess from the 20:1 reduction. I soaked everything in a carburetor cleaner bucket, washed everything in the sink with a wire brush, buffed up the jaws a little to remove raised rust, and tomorrow I should be able to press it back together.
I have a feeling this is a very old chuck, and it doesn't have much identification as far as jaw positioning goes. I'll have to play around looking at the thread offsets to find out where they go in relation to each other. They don't have the progressively deeper grooves that Jacobs points out in their assembly guide.
With luck my next project will be a 0MT to 3/8-24 UNF arbor for the AA109.