Based on the testing boywonder and I did, it is my feeling the louvers on the hood do nothing to “extract” or introduce more air into the engine compartment while moving. No question they allow heat to escape when stopped.
Extractor vents on the fenders might do something while moving and would help with heat escaping at rest. We did not test that area but it seems logical it would be a lower pressure area across the outer sides of the van front fenders than underneath the hood. Keep in mind there are other considerations of under hood airflow such as across the exhaust manifolds and top of the ttransmission.
The reverse scoop idea at the rear edge of the hood probably would not create much extraction either, it is a high pressure zone when moving. (edit:Just saw carringb’s post after posting this, same idea)
A front facing scoop behind the radiator would increase pressure behind the radiator and would likely reduce airflow through the radiator. I would be very reluctant to do anything that could potentially reduce the airflow, while moving, through the radiator and other coolers that face forward behind the grill. Creating a lower pressure behind the radiator would be helpful. The air deflector/dams that hang down below the bottom of the radiator would help do that.
Just my two cents