Also, though it will need testing to see if it's actually viable, I've been able to modify things such that:
- if you are editing an assembly
- you remove a material from the scene
- the material is removed from the assembly
- referenced part documents are searched
- the material is also removed from each one found
I still have to consider whether this will bring any unintended consequences (i.e. multiple assemblies opened, which reference common parts; also, assemblies with sub-assemblies, etc.).