
Somewhere around here there's a link for Thor that shows a guy that hooked up a flamethrower to his trombone. I think it might've been Thor, come to think of it....
CC LinkNot to be picky, but that ain't a euphonium. It looks like an EEb tuba to me, possibly an F tuba. (Don't worry -- all those horns are related. There was and is a lot of incest that goes on....

)
Something like that could only be done, well, very carefully. Can't do it on a silver-plated horn and you can't do it on a lacquered horn because the lacquer would burn off. The silver-plate would discolor and probably melt.