I don't know if you caught it, but the most recent post from kach22i in the
Random Wind Tunnel and Smoke Pictures Thread Tells the 'tail' of the Porsche 917.
The proportions and goals are different, but maybe there's something to be learned. What your hinged plate reminds me of is the Dauntless dive bomber dive brake.
http://www.davidpride.com/Aviation/MAPS/images/MAPS_03_090.jpg
What is the use case? Is this a high-speed vehicle that deploys the flaps for landing?