Sounds crazy, but one of the best videos that I've seen that shows this effect is a Mythbusters episode where they covered a Taurus in clay and then dimpled it like a golf ball. Don't remember the entire video, but by dimpling the car, they increased / decreased (I don't remember which) the air gap behind the car and actually increased the gas mileage. There is no way I would ever have thought this to be the case as I had always assumed a slippery design would be more efficient. My point is that the "air bubble zone" behind the car should be understood more - especially if we (I) want to flow more air through the engine bay.
I've seen the various videos with the strings on the sides and back of the car and now on the back of the a/c vent area, but has anyone checked the airflow in or out of the air screen / off the back window? To your point, the inherent design off the back window would cause this same effect to a degree - correct? So what does that mean? Does it make the air more neutral so that it could be moved in or out of this area (depending on need) OR does it already create a suction in a given direction?
The reason that I'm asking is I'm not sure which way the air is moving through the engine screen and if you could create a draw or suction that draws air into the engine from the top, it could alleviate some of the dust / dirt issues drawing air from the bottom of the car potentially creates while still cooling the engine. Based on the heat build-up I've seen, it doesn't seem like there is a given air flow direction in the engine bay, but my expectations may be unrealistic.