Technically not supposed to be able to do that, but I'm all for bending the rules when the situation warrants.

I had a Drunken Master in the game I was running once who grappled down an enemy wizard and proceeded to beat the monk with him. I counted it as an improvised weapon, and he was doing quite a bit of damage with it. I ruled half-damage to the wizard. Pretty much ruined the encounter (what with the failed Will save sending the cleric fleeing to the toilet), but it was fun.

It also set the precedent of the Drunken Master using enemies to club other enemies to death.