Maybe you can, but it probably wouldn't be worth the trouble.. I imagine it takes in different variables that it takes into account like, distance the face is from the phone, the size of the face, certain details, etc.. So I'm assuming the only way to "fake" it of you print a life sized picture of the face, or use a computer screen..
That may not work either, as the proximity sensor (using infrared) may be advanced enough to figure out if its flat ( paper) or not.. I'm just guessing though..