I was gonna just blur it, that was my original plan. But I was thinking about a more 'cops' style censor, or whenever genitals get shown on TV. It's just a basic mosaic filter, but FCP doesnt appear to have an equivalent.
now, to mosaic just a face is kind of a pain. you need three tracks. on v1 is your footage. on v2, use the fcp "shape" generator to make a white, face-sized oval with a black background.
on v3, put a duplicate copy of your footage and apply the mosaic filter to it.
on the v3 clip, change the composite mode to travel matte-luma. v1 and v2 stay with composite mode as normal.
the pain comes if your bad guy/ person who didn't sign a release/ genitals do any moving. you have to manually keyframe the oval moving to match your blurred subject. because fcp doesn't have motion tracking. if you need to do a lot of this, you can get a motion tracking plug-in for $50 here: http://www.lyric.com/fcp-plugins/