I wouldn't hold your breath for anything ninja-specific using the throwing slot unless its a shuriken. From the Sange update it's pretty clear they're trying to get Ninja's to use them more and something like armguards would pull away from that.

Besides, there are plenty of items which can be equipped in the ammo slot anyway. It doesn't need to be a specific 'new type' of item to offer stat bonuses in the ammunition slot. We've had stuff like that ever since the Morrion Tathlum.