Playing around in pixel bender, trying to enhance deface, I've created a skin color filter for a prepass. Unfortunately, it didn't seem to actually reduce the amount of time it takes to re-acquire a lost face. Oh well, it's still useful, so have at it. I've put it up on adobe pixel bender exchange.

It works by translating the image into YCbCr space, then applying a simple thresholding on the Cb and Cr components. The technique and parameters are described in this paper.