Ability to make anything local

As of 2.78 you can't make an object data or material local as long as the object is linked.

Here's an example of why it can be a real problem (but there can be many other reasons) : Let's say I have a big complex scene for which I have to do several different material passes. Not just render layers with a single material but have several materials in each pass. To do that, I could just link all the objects in another scene so I could make only the materials local and change them. But I can't. I have to make everything local. So then if I want to make changes on the objects (move them around, edit some meshes), I have to do them as many times as I have passes.

If we could make a material (or data) local while keeping the rest linked, any change we'd make on an object would be updated everywhere while our pass materials would stay different.