Adobe engineer Pat Wibbeler wrote me today with a good suggestion:
The idea is simple: Instead of cloning the region exactly, clone the mirror image of a region. I wanted to do this when repairing an ear in a recent photo. I’d like to have simply cloned the opposing ear in reverse. I accomplished this by copy, pasting, reflecting the “good” ear and then cloning from the mirrored copy. It seems like it would be pretty straightforward to do this automatically and that it would be useful for other applications as well.
What if, I replied, I told you the feature was already in Photoshop, as of CS3? You’re pretty much guaranteed never to find it, though.
Open up the Clone Source panel, then specify a negative number for the width value (e.g. just put a minus sign in front of the “100”). Now Photoshop will flip the source data so that you can clone a mirror image. You can also use the panel to scale & rotate source data without having to select/copy/paste/transform it. This all works with the Healing Brush as well.
For more info & to see the technique in action, check out this video tutorial from Russell Brown.