As far as ducting goes, my car (TDI 150 Sport Toledo) has a duct from the front bumper (below the foglight) to the side curtain by the gearbox. I can only guess that, as the diesel undertray goes all the way back to the floorpan, the ducting makes sure the gearbox gets some cooling.
The LCR undertray has a couple of ducts but I'm not sure where they end up.
My old TDI 110 had the full-length undertray and no ducting.