California98Civic,
Thanks for bringing this thread to my attention.
I saw this flat spoiler on a diesel-powered Metro at the 2011 Green Grand Prix. The rear spoiler thing is a clear plastic panel, supported by a simple bracket at the center. The car is an X-Prize entry, by BITW Technologies. It scored 106 mpg at the GGP but I have no idea whether the rear plastic panel helped or hurt.
I'll put the pic here, and couple others of that car.