...Newey built the RB7 based on the premise of getting more air "(blown exhaust) underneath the car, so maybe this is a replacement for that lost air...
Probably not. You see, once the car slows for a corner, that air flow would taper off. The blown exhaust allowed for high speed/high volume air to go into the diffuser despite the car having slowed down (which is why the throttle stayed high even with the driver off the throttle). This is similar to the boundry layer control device or "blown flaps" of some second generation jet fighters in the early 1960s, which used bleed air from the engines to blow over the flaps for more lift at landing speeds.
Perhaps it is some kind of weird take on the "f" duct or whatever they called that device from the 2010 season. Perhaps it adds to airflow at higher speeds, or leads to another radiator, etc. Certainly will be interesting to see what it's for!