I've never tried to stamp my M&P but surely it would work. I stamp all my CP soap and I dip my silicone stamp in Mica(you can use a q-tip to clean the edges of the stamp so that the mica is only on the writing part of the stamp) then place on your soap and lightly tap it...i use a very small hammer. I'll try this on my M&P and let ya know how it works. Good luck. Also, Soap Queen has a video on Tattoos and M&P.
Lara