Well, I just found out (or rather, ran into and was instantly reminded) that CC2 did in fact give it a use in Redemption since apparently they had no idea what to do with it otherwise. It's in the battle Haseo watches with Sirius of three cannon fodder fighting Taihaku. The file m30620_s01e.tmp has copies of the graphics for Maxwell, the Golden Spear, Broad Demon, and the mystery weapon. Whoever the Steam Gunner of that party was is the owner of this weapon.
I guess there simply wasn't reason to use those graphics prior to Redemption... or maybe I should actually look at the other games' data and find out before I assume things? After all, if they went to the trouble of erasing things between Rebirth and Reminisce (namely Ovan's, Gabi's, and Matsu's Arts (broadsword only for Matsu of course)), then Rebirth might have a different version of it; the one in Redemption was (now) obviously a default setting by whatever program was used to set up weapons so the graphics could be used in that fight.
Weird that they went to the trouble to build that weapon for one cutscene when they could have just used the Gun Steel or something. Did those guys happen to have names or anything? :\
EDIT: Not worth making another post for this...
I found out who uses it. I did a full write-up for my little side project documenting dummied data and gaming artifacts that just barely saw daylight:
The weapon belongs to drop_point, a tres-lookalike LV. 108 Steam Gunner who fell victim to Taihaku in a Sage Palace title match that Sirius invited Haseo to watch early in Redemption. He is part of an Arena team along with Hickson, a LV. 109 Edge Punisher that looks like Kaki Leader and wields a Broad Demon, and ryokichi, a LV. 109 Lord Partizan. The Mark imposter uses a Golden Spear and is the team leader. drop_point is the player that makes the mistake of using a Lei Zas-like spell against Taihaku, letting him show off Maxwell's Null Magic property, so the unknown weapon never gets any actual use.
In a bit of trivia, ryokichi and Hickson switch weapons and classes when the fight begins and switch back when the spell fails, making this team, only named "Taihaku's Opponent" in the ELF file, that much more special for the apparent ability to switch between each others classes and weapons (or in other words, the event programmers screwed up who was who). ryokichi is voiced by Yuri Lowenthal (sp?), while the other two PCs' VAs are currently unknown.