I can seemingly change Curse to a Ghost-Type. Then I can edit the ???-Type graphics. I can also use YAPE to change a Pokemon's type to ???. I can also change attacks to the ???-Type and I've read that the ??? attacks does damage as it should as long as the Physical/Special Split is implemented. The only thing now in theory, would be to edit the ???-Type effectiveness because from my testing so far, it is neutral to everything. I would need to find the pointers for that and look a bit deeper. Editing type effectiveness in general would be great as well. I can make some adjustments to types that I felt that Gamefreak should have done.
Well, I'll screw around making some Pokemon/damaging attacks ???-Type and see what happens. In theory, this should work out.
EDIT - ??? gets STAB attacks and does damage it is supposed to... so all I would have to do is edit its type effectiveness... hmm, seems to good to be true.