Same can be said for X Accuracy + Fissure/Horn Drill, I'm quite sure this wasn't GF's intention either.IMO it's not even a question that using items on full HP Pokemon wasn't Game Freak's intention. Glitch/bug/whatever you want to call it.
grinding in Pokemon Mansion isn't part of the question since you do have to go through it anyway, so why not use it to train Omastar?
yes i admit rock type isn't everything but Omanyte takes only 10 levels to evolve, something that is piss easy to do because of Pokemon Mansion/Blaine. He also ends up stronger than Blastoise too.