Linking group with individual controlling empties

I have a model set up with an empty’s position controlling the material. See attached blend file. The sample model there is a simple object and an empty put in a group. The final object of course will be composed of more objects, but this should be sufficient to demonstrate my issue.
When linking the object in another scene I can make the empty a proxy object and control it in that way. However when linking the same group multiple times all proxied empties behave the same way, e.g. when moving one, all the others move as well. The effect is that there are no individual materials.
There must be a way to set this up so that I can control the material of each group through it’s empty for each group individually. However I cannot figure out how. Any ideas?

Attachments

MaterialHorizFade.blend (634 KB)