What I would do is flip the skeleton before you make the animation, if you want to do it during the middle of an animation then its not the easiest to do it in a 2D animator. but you would make half of the animation, then flip it, then make the other half.

You can flip the character at runtime. We'll have a button to do this eventually. Just scaling negatively on X does not do the job due to rotation and the way parent/child transformations are calculated.