I am by no means an expert in aerodynamics, but this is what I would attempt(excuse the very crude drawing)
On the spoiler I have drawn a rectangle. This would be the hole cutout. In the very crude hand drawn thing below, this is my idea. The spoiler bolts onto the bumper right? Ok, the red line marked as 'block off' speaks for itself. This would probably need to be manufactured out of a nice rigid material. There is a mini spoiler right under the bumper to direct air into the newly cut hole(this could probably be disregarded anyway as the air would dam up under the bumper anyway).
The way I see it is that this way there would be enough air ducted to the oil cooler, but in my humble opinion kinda defeats the object of having a Kamei no? (ie preventing air under the car in the first place). HOWEVER, I do see some logic behind it. Some air passing to the radiator is better than having air passing under the whole car
(again sorry for the crude drawing! lol)