Hi Richard, saw this the other day but got side-tracked- you may already have the adjustable flame 'Mappgaz' do-dad....much more versatile than the original, though I haven't used mine for brazing, but it isn't actually the authentic Mappgaz one....Mappgaz ain't what it used to be either...!
In the case it doesn't work so well, but am sure it has, l I've used 'Devcon Plastic steel' on (and it goes against my grain to do so) my Gutzzi rocker cover for emergency fix on a big hole after I went down the road, and also the fuel tank on it, and my Hilux the other week.
The rocker cover was ten years ago, and seems to work ok- thanks for the reminder, totally forgot about it and have had it powder coated in the meantime.....so I guess it must be ok for heat