[Christ]

Jim -

Not answering for Robert here, but the vent behind the plate may work insofar as being able to vent air into a low pressure zone... however, if the smallest cross section is only as tall as the plate itself, you'd want to prevent any flow from occurring top or bottom of the plate, and only allowing it to occur at the sides, which could train the separation at the sides of the rear transom.

Easiest way to do that would be to mount the plate about 1/2" away from the transom with bar-type spacers top and bottom.

Placing a vent anywhere that the pressure difference across the vent is higher than OE will improve the performance of thru-flow heating/cooling and defrosting, with the usual diminishing returns clause.

EDIT - Woops, I forgot to say why you don't want flow above and below the plate...

When the rear transom is only the size of the height of the plate, in this case, six inches, the flow will separate at that cross section. Any air vent which causes flow to exit above or below the plate will increase the effective (dynamic) cross section of area that the flow sees.