In the GRA, the Katana apparently has a mod file where it has a scabbard. When I first saw it in screenies, I wondered how exactly they pulled that off, since all the known ways of getting holsters and sheaths and whatnot was through rather convoluted methods.

Well, it seems it's done with the GRA Katana by making a separate Backpack node, with a NiStringExtraData that makes it bind to the pelvis bone. Thus, both the blade itself and its scabbard are in the same nif; no armor addon, no separate apparel item. Some positioning tweeks would have to be done to get it positioned right, but one could use the GRA Katana nif as a guide.