My very first reaction would be to say that you've stated a counterfactual... rape will never directly produce more utility than disutility. So the only way it could be moral if, somehow, unbeknown to us, this rape will somehow prevent then next Hitler from rising to power in some butterfly effect-y way that Omega knows of.

I have to trust Omega if he's by definition infallible. If he says the utility is higher, then we still maximize it. It's like you're asking "Do you do the best possible action, even if the best possible action sounds intuitively wrong?"