Pixel'd face effect?

Use the mosiac effect, under stylize in the effects menu. Make a dupe of the layer, mask the hidden part, apply the effect, and tweak to taste.
 
Instead of duplicating the footage layer...you should be able to add an adjustment layer to do the same thing. I haven't tested in a while, but formerly when I did similar things, the adjustment layer vs. duplicating rendered faster.
 
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