Well there’s a good reason why there isn’t a true Xref plugin : it is not possible in SketchUp.
This is the same reason why each render engine has to implement its own proxy system.
Skatter is not a render engine, it only passes the scattering data to the render engines.
The fact that there are proxies for Vray/Thea/Maxwell/Enscape in the Library is because the components contain metadata that link to them. It is not related to Skatter itself.