Hacking the item number in won't work, because the data for that item doesn't exist until you obtain it the "proper" way (get it from Nintendo or someone else)

Presumably, the hex value for the item would be stored within the item data somewhere, but I didn't notice it right off the bat. We could create "custom DLC", but to avoid conflict with official Nintendo DLC we'd have to set the hex value to a really high number, or some other number Nintendo won't logically use.