This was not possible when Library Overrides were implemented, however much time has passed, but I have not seen anything regarding this, maybe it’s still not possible because I remember there was some quirks about material datablocks.
Its not about rushing or asking for the feature, just for the record right now we can do the material override using geocodes and assigning the materials with a modifier that is perfectly overridable, but could be good to know if this is present already or not.
Thanks!
