heat will radiate out of the open gap for sure, but it won't allow air out once you're on the move.
reason being, due to the change of gradient at the windscreen, the base of the screen is actually a very high pressure area. Comparatively speaking, due to the small area of frontal openings into the engine bay, the engine bay is an area of low pressure, so air will actually come IN through the gap at the back. That's why many cars with bonnet vents at the back of the bonnet face it backwards.