I have a part definition, to which I want to attach a material with a Geomety Map defined. When I look in the Render Properties of the Family/Part editor, I see a Render Palette can be assigned. When I go to RMC > Properties, I can open the Material Editor, in which I can open a Palette to see the materials available in dgnlibs. I can go RMC > Assign To Part, which gives me the option to assign it to a part.
I notice sometimes the material seems to be coming from a dgnlib file (see image 1), sometimes from the local file (see image 2). I don't seem to be able to edit this in this menu. It seems once the material is somewhere local in the file, it refers to the local file, which I think you should avoid in the Part libaray (since they are for all projects with that Dataset).
Image 1:
Image 2: