I have a solution.
Your hand is a light weapon for a size class of whatever you are. So for a human, a hand is a medium Light weapon. Now, you need to increase the size class of your hand so you can make it a two handed weapon once your hand is Huge.
Of the top of my head, the Kensi PrC allows a monk to put weapon abilities in your hands, so you can take it long enough to put Sizing in your hand. Then, any time you want, you can increase the size of your hand two sizes, then you will HAVE to wield it with two hands, so you can use power attack with it. Plus, it has the added benefit of increasing your base damage, at a -2 to hit per size class increase, unfortunately.
That said, I would never allow this because I know one certain player of mine would state that "any body part is considered a weapon for a monk" and lord knows he'd want sizing in his penis.