Your muffler looks really good and Scotch Brite on the outside will surely not hurt it. My information on Scotch Brite comes from an engineer at the Cadillac factory. Using Scotch Brite leaves very small particles that if not properly removed do what abrasives are meant to do and if that is the inside of your engine, well . . .

Rubbing compound and polishing compound could also advertise themselves as not causing scratches I guess.