I just checked out how to put all the materials into the library and it is actually much simpler than that. – All that has to be done is copy the .blend files into the library folder and all of the materials in the scene will show up in the material library. The name of the .blend file becomes the category name.
So all that will have to be done in the end is to clean out all unnessecary materials of the scenes and copy them.
… We will have to see if we even keep the blender internal and other materials in there? – I would just go with a clean library, showing only the different material categories.
Can you have a look if it is possible to alter the addon in a way, that the preview is automatically updated when the user clicks on one of the materials? – Right now the addon requires to always click the preview button, which is quite annoying. – An auto update would at least be a step in the direction of faster visual browsing the library here.
Also some bigger and more understandable icons on the sidebar of the addon would help usability.
EDIT: Would it be possible to call the material preview from BfA into the list of the addon? That way a dynamic list with previews could be generated. – Or else, would it be possible to include images in the list? Then we could prerender all of the materials to include a preview that way.