Nice job, Michael, but as your example elegantly demonstrates, there’s no magic. The shadows and highlights have to be there already, and it’s really just a form of posterization that the OP wants. The thing is, such treatments, plugin or otherwise, produce the effect mathematically, and can’t possibly reproduce the artistic subtleties necessary to emulate the touch of Michelangelo.
Another recent, "gimme a plugin that does this" thread made mention of the good ol’ KPT Equalizer, which could be the best plugin ever for creating such effects, but it’s no one-click affair either. A lot of patient experimentation and multiple selective applications might get you close.
