I have had similar problems especially with proxies and wrote a blender plugin. Maybe you can also use this, The plugin is a beta-version but I work with it.
It is available for blender 2.79 and blender 2.80 (2.81 I did not test yet)
https://github.com/black-punkduck/MakeHuman-Helpers/tree/master/blender2_7https://github.com/black-punkduck/MakeHuman-Helpers/tree/master/blender2_8The software works with a so-called mirror-table. It can be used when you export with mhx2 or other tools which will not destroy the vertex order of the body.
In this first step a mirror-table must be created (e.g. by using the standard body) and must be saved (it is a simple text-file). This table should be assigned to an object (a valid character using same topology) later.
This table can be used then to:
- to select mirrored vertices
- create a fully symmetric mesh
- mirror vertex groups ("weights")
- mirror shape keys
it works also on a non-symmetric mesh which is useful when e.g. you want to use corrected weights from one side on the other one.
It was used to create weight-files for the MakeHuman skeleton (Bento-like) and is also able to import/export mhw files
I added a manual with illustrations:
https://github.com/black-punkduck/MakeHuman-Helpers/wiki/blender279weightplugin.mdMy last blonde girl was created with a special own proxy and different weighting for bones and (blender-)shapekeys also. I did that with this tool. And also her cat
http://www.makehumancommunity.org/forum/download/file.php?id=7444&mode=view