you can attack a small fishing line weight to a small gauge wire.. and slid it down the exhust tube.. and work it back and forth to break up some of the built up carbon.. then I would suggest takeing a small rag on the end of the wire and pulling it through the pipe several times. Once you have no more nasty comming out... or if there is still a huge amount i would actualy pour carb cleaner form a vat.. aka kerosene and let it soak in.. then repeast the process..
I don't like the idea of heating it up.. the carbons already been baked on everytime you ride it.
and eventualy your going to have to do some scrapeing and shaking anyways. on puegeots the pipe has an aliminum casing that will actualy bend and warp and is very toxic/ If you exhuast is aluminum I would drink a carton of milk first just as we were taught to do when wleding aluminum and wear a mask.... or do my way and don't heat it.