I do not believe either that a tsuba with a "square" nagako-ana was intended to be mounted on a naginata. Afterall, the cross section of the tang on a naginata, yari, katana-wakizashi-tantô are of the same shape --> triangular.
I have a Norisuke tsuba with a square nakago-ana, it is obvious that the tsuba has been rotated 180° to suit the fancy of an owner, but in no way was it intended for a naginata:
In "The Picture Book of Old Tsuba" there is a Negoro tsuba that was probably mounted on a nagamaki-koshirae. The boldness, the flat square-raised mimi and the lack of kozuka/kôgai ana do seem to point towards a tsuba specifically dersigned for a naginata (sadly there are no dimensions given:
Christian, thanks for those 2 PDF's, I also have an interest in Bannermen !
Paul.