Heh... any piece of a 36 HD Pit Fiend is hardly a "mundane" item. And it is to be used to make a Simulacrum..? I'd say that this Wish has to be GRANTED by the DM, with all associated risks and interpretations. But that's just DM fiat for Wish.
Heck, in PF, Simulacrum doesn't even require a piece of the creature to be duplicated.. just ice or snow.
Still, I want to be behind the screen when some hot-shot conjurer of simple tricks pulls this one out of their hat...