I am trying to think of a notch back that has the rear of the body work within the 28". I know all mid engine cars have and the engine lid itself eats up the majority of the 28".

since the pillars of the cars come back and the rear window is vertical.. this creates a large high pressure area thus making it worthless for any wing on the car.

as you can see here.. Air becomes turbulent at low speed.. at high speeds it is even worse. The wind tunnel testing from I have found was performed at 60 kph (~37mph)

The CFD picture is the data from a I believe 100nph.. you can see that the air does not recover.

So for all the mid engine notch back cars they would need to have the rear wing to be above the roofline as well to be effective. Which I think was the point of adding notch backs to the list in the ruling.