Don't heat the capstans, then. Heat the wire. This should be a lot easier than heating up the metal capstan.
I looked up some old records of my first rebuild after school on my family's 1878 Hallet & Davis upright. It had stickers attached to keys with a dainty turn-buckle arrangement and lost motion was adjusted by turning a small (1.5 mm) screw between parts with opposing threads.
100 year old wood, (this was 1977) and frozen threads. As a newbie encountering this antique, I broke the first one, of course. I broke the second when I tried lightly twisting back and forth. I was a tooled up Toro in an unsuspecting china shop.
I had no torch. I had a box of Ohio Blue Tip kitchen matches, (100 of them), so I tried heating the threads and managed to set the third one on fire. Man, that old maple sure likes to light up. But, I had aluminum foil and crafted a heat shield that looked like a 4 year-old's version of a dinosaur on a volcano. It worked out to about 2 screws could be heated on one match if nothing else caught fire. I don't know why SPM never caught on as a standard unit, but you think about such things when there are endless screws to cook stretched out on the bench and a dwindling pile of kindling.
The beauty of the wire is that you can find out by trying to twist the capstan as you heat the wire, just how much is needed to loosen the bond. If it is so hot that it smokes, or you see flames between your knuckles, maybe a little less.