There is only one way to clear a clogged muffler with out gutting it. This is REAL fun, and REAL dangerous...
You need an acetylene/ oxy torch. Start at the cylinder end heating the pipe to cherry red. NOT blue or white, RED. As it heats you will see sparks of carbon start to fly. When this starts turn off the Acet. and run pure Oxy. into the pipe. It will keep the carbon burning, and you will see the pipe glow in a ring moving towards the outlet. It moves back until you pass the baffles, and will SHOWER EVERYTHING in it path with burning carbon. (that's the fun part). I would HIGHLY recommend a Trained welder to do this for you. Chances are if you find a guy that has done this he will do it for the cost of the gas as it really is a show to see.
Or just hit ebay or OEM for a new pipe. There is nothing that will "wash" it out.