I just went throught this with the Ironman I purchased. We did the following: The quill broke loose almost immediatly so that's not an issue. The LBS soaked in PB blaster for several days. Upon trying to remove the stem by turning with bike in a vise, the stem broke off at the head. LBS drilled out the stem as much as safely possible to relieve pressure. I soaked it for one day in Cola, then one day in ammonia. The ammonia had a visible affect on the alloy stem changing the color to a flat gray color. Next I took the fork with the stem in it to a metal shop. They cut a wedge in the stem with a metal chisel to relieve the pressure. Then heat was applied to the fork steerer BELOW the threads, NOT on the threads. With the fork in a vise, and the stem in a vise grip, one guy (me) holding down the steel table holding the vise, another man turned the vise grip and the stem budged. ONLY AFTER THE STEM MOVED considerably did we stop trying to remove the stem from the top and used a punch to drive the stem from the steerer. Using a punch to drive the stem from the bottom is risky because the quill wedge could simply jam the stem making it even more difficult to remove. In fact, when the stem started moving the quill jammed once on us and we had to use the quill bolt to tap it loose. Then we finished the job pulling the remainder of the stem from the steerer with vise grips.
The remnants of the Nitto stem are in the pic below.
I would not wish this problem on my worst enemy. Well, maybe my worst enemy, but not a fellow BF member.
I hope you achieve your goal. I'll share any information I can with you about this experience.