I remember watching a old hotrod builder doing this on TV many many moons ago. I knew then I had to try it.
I tried that lead alternative once (triying to be eco friendly)...hated it I found it was hard to work with compared the the lead based filler.
That is awesome on the vintage tools. So many are better built than just about anything today.
I am no expert for sure, but I managed to fix and fill my rusted roof gutters (split at the seam) on my old Bronco and then various spots on my old Chevelle (windows channels etc). since then I have used this method every chance I can get.
Like you said key is to keep the heat down, but keeping enough to melt the led. It is an art that I am still working on.
People I guess are afraid of it as you need flame and are melting metal, but once you get the hang of it, it is rather easy and the end result is awesome. Smoothing it down is no more difficult than fiberglass filler or Metal (aluminum type) filler, you need time and patience. The result is once blended in it look to be part of the metal.....just shiny and as strong.