I am pretty sure there are some problems
For example: where do "hands" stop or begin ... Everybody would have a different opinion. Then we need to do it for all proxies etc. etc. etc.
But depending on the application used after export there would be simpler ways to do it. Given the fact that some formats keep the vertex order it would work for the body at least. Because it is a standard no matter which targets are used.
It can be done for proxies as well.
Technically it would be a simple table, which can be connected e.g. in Blender. Simply a name + all vertex numbers for the "feature" hand.
In a way it is already existent in Blender. There it would be a vertex group which can be transferred from one mesh to the other. For other applications I cannot speak. I use exactly this method for a new weighting (e.g. if bone deformation will work more with no volume or with volume), where I can copy the group from one character to the next one.