I just created a lot of shapekeyed facial expressions on a head/neck with appropriate head/neck weights, weights that I want to keep. I would like to create vertex groups like mouthCenter and mouthLateral so that I can play with limiting the shapekeys to various parts of the face. I would like these groups normalized to each other so that I could, for example, apply SmileLateral and smileCenter both at 1.0 to create the entire shapekey.
However, the normalize all tool from weight painting won't work for this, because it will normalize my (armature) deforming groups along with my non-deforming groups. Even if these groups are locked, it will normalize around them. I've tried using the Shapekey Helpers add-on to apply a data transfer from a different model, but it doesn't apply the data transfer (when using its apply modifiers and keep shapekeys option from the shapekeys menu.) The shapekeys appear to be applied prior to modifiers, so I believe I have to apply any data transfer I get for weights to restrict the shapekeys themselves.
I imagine the same would go for a vertex weight mix, even if I were to figure how to use that to normalize vertex groups to a limited subsection of all groups.