Shot sculpting is pretty common practice in animation. Whenever the rig needs a one off fix for a specific pose in your shot, you just create a blendshape to change the geometry accordingly.
But since I can’t edit a linked character, and creating a proxy would break the vertex groups, how would I do that? Or is that not possible?
So combining proxy and library override doesn’t work.
Using only library override works, as long as you add an additional override to the mesh data.
However it doesn’t seem to quite work in 2.81. I can’t move verts in edit mode and sculpt mode does weird stuff. Hopefully this will be fixed in upcoming versions.