Linking materials with groups and drivers

Question

I am working on a project setup and trying to understand how it will all work behind the scenes. It involves several layers of linking, material groups, and drivers.

Can I have a value in one file drive a driver on an object linked in from another file? For example: If a material on an asset within it's own file has a driver based on some custom value of a scene, how can I have it take that value from the scene it is linked into rather than the scene within it's own file?

Or, inversely, if I setup a driver in the main file, how can I have it drive the appropriate parameter of objects that I link in?

An example of this would be if each of my materials or material groups had an input for wetness. Different materials are effected differently, and will need to be setup within their own file. These assets are then linked into whatever scene file I'm composing a scene in. A rainy scene will have a wetness of 1, and a dry 0. That needs to properly connect to each material's wetness input and drive it.

EDIT: This question was originally too complex and involved several questions, I have simplified it.

Answer 1

The limitations of Blender's linking system make this a little awkward, but you can make it work with the proxy system. Proxies are Blender's funky solution to editing linked properties. You can "make a proxy" of a certain object from a linked group. This makes a local copy of that object which will then push any changes into the linked object it was copied from. Here's generally how it's meant to work in practice.

Your asset has an armature in its source file. This armature object is set up to control all other objects and materials in the asset via drivers, constraints, parenting, modifiers, or whatever. You pack everything into a group, then link that group. With the group selected, you hit Object > Make Proxy in the 3D view, and select your control rig from the object list in the prompt. You now have a control proxy that you can move and manipulate to control your linked asset.

It doesn't necessarily need to be an armature, you might just use an empty in this case. But essentially what you need to do is connect your material's wetness in the source file to some property of this empty/armature via a driver. A custom property on the object datablock is probably easiest:

Once you make a proxy of your control object, you will then be able to adjust this "wet" property from the main scene. Word of warning though: it will affect ALL instances of this group in the scene, not just the one you had selected when you hit 'make proxy'. Best way to get around that is to add a symlink to your source file, then link the group again by select the symlink. Blender organizes links by filepath, so it will believe this is a different asset even though it's reading the same one. You can then make the new linked copy its own proxy and control them separately.