Ok, i have had a look through the item list, and i cant find anything out of the ordinary (apart from a few unused key items).
This means that Game Freak simply stuck the code for curse arceus in to prevent the game from crashing, since the ??? type move is basicly another move type.
The only way i see curse arceus seeing the light of day, is to take another plate, and modify it (action replay or not) to be the ??? type.
for example, you take the spooky plate, and change its code to use the ??? typing.
I cant exactly test this at the moment, i need to brush up my action replay code skills first.
phew, first post over and done with...