Beating a dead horse(man)
Small update. I kept beating the horse, sorry.
A few days ago I put a horseman down with a little tool called matbridge.
The thing only knew Poly Haven materials, and I wasn't happy with that. So now it does generic folders too. Point matbridge at any folder of textures on your disk and it builds the same shared library out of your stuff, matching names in Blender and Unity, nothing to wire up.
Same trick as before. Just with wider support.
Still right here: