Yes, flip might have been a better word. Despite all this time in English-speaking countries, I still struggle with the language all the time. To clarify, here's what I wanted:

Input on the left, flipped version on the right, sum of both in the middle. Note that the middle does not simply mirror one side of the input, because the morph influences both sides of the face. Also the x delta amounts are quite different for the left and right side of the face (her right side basically not moving at all in the x-direction), so ADP's script wouldn't be much help, even if there was no movement in y and z. What I did instead was spawn two copies of the input morph, mirror "-x to +x" for one and "+x to -x" for the other in the morph tool and add those together to get to the image in the middle. Then I subtracted the original morph to get to the one on the right. Really not that bad once I'd figured out the technique and could streamline its execution.

An alternative would have been to export the mesh with the morph applied, flip the whole thing (not just the deltas) along the x-axis, then use my vertex numbering repair script to get the flipped morph from that. Good method if I'm making morphs in Blender anyway, but here I was actually just tweaking an existing morph within Poser, so staying within Poser seemed easier.

-- I'm not mad at you, just Westphalian.