it's simple, have the monk harden and sharpen his natural fingernails to the point that they are stiff enough to pierce flesh, and then he can choose wether to punch and deal bludgeoning or jab with his fingers out straight and deal piercing.
This reminds me of an attack that one of my characters used in a anime-style fighting game, you might have heard of it, it's called
Thrash. My character was a 6-foot tall cyborg powerhouse (aptly named Bouncer) whose favored attack was a "Fingertip Attack", which never missed, dealing a flat 1+STR BONUS damage. His strength bonus was +15 damage....

. The worst part was that he was able to use this thing up to seven times per turn!
*Bouncer walks into ring with Rock Hauer*
Rock : RE-PU-PLEGHM!!!
*bouncer stands there and takes the blast, dusts himself off*
Bouncer : POKE-POKE-POKE-POKE-POKE-POKE-POKE! *walks away as a janitor enters the ring to clean up the pile of blood and meat*