sinse theres many ways to mod weapons, i always wondered if there was a mod to change a weapont omasamune, then replace this novelty item with the actual one. wouldn't know how to do that though
You can allow the masamune to be equippable by multiple people then make the ride give you the weapon as a prize instead of the item. Problem is when you equip it on someone it'll look like their default weapon (although it should SOUND like the masamune

).
You could also modify the battle.lgp to add the masamune model to the characters' weapon models. That's a bit more complicated though and they'd look weird holding it like any other normal weapon of theirs.