This MCG will mirror the form of your geometry across an axis for creating morph targets. The Form/Shape is mirrored, not the geometry itself or vert IDs as this would render the targets useless. The geometry's form/shape should be symmetrical (pivot centred) but vertex distribution does not have to be symmetrical, as can be seen with model with one eyebrow. Modify your target and the mirrored result is updated live, not like some static, dead end mirror/skinwrap method.
Requirements: Max 2016/17/18. Note Max 2016 also needs Extension
for Max 2016, please apply Service Pack 4 first:
then apply the extension 2 (you have to pay Autodesk extra for this, I can't do anything about this unfortunately).
Note, below is info link, not download link:
Performance in Max 2018 is very good, however for 2016/17, it can be sluggish for high res geometries, it is advisable to simply disable the modifier while you tweak the target, once happy, then you can re-enable the MirrorMorph modifier.
Note: This requires the availability of the mesh in its rest state, ie. a copy of the mesh with no targets/deformation applied.
ver 1.01: added Combine mode so you can see combined effect on both sides. This mode should not be used with Blend