In theory, if air passes over a vent, you create negative pressure and it extracts. If the air pressure at the bottom off the vent becomes more negative, then air gets supplied down the vent.
In an engine compartment, the air gets dragged under the car so at or near the bulkhead it would become a vacuum so, if a vent is introduced at the rear of the bonnet it should (in theory) help air flow downward and remove the heat under the car.
The only problem is, Which pressure is greater? Under the car or over the bonnet?
Nip into your mother's knitting bag and cut several four inch long pieces of wool.
Tape these to the bonnet at various points and drive. Watch which way the wool moves to give you the correct info.
This was how aerodynamics was done a few years ago.
Cheers
D