We tried this on a family road trip a few years ago. We tightly wrapped leftover pizza and chicken in individual foil and placed on the exhaust manifold of our Dodge Caravan. After 100 mi. or so. we stopped for lunch and had a hot meal.
I'm looking for road trip recipes for the 911. Unfortunately, the muffler isn't too accessible as a heat source. However, early heat exchangers produce about 400ºF surface temps. I think a sheet of aluminum, bent to fit and attached to the heat exchanger, right before the collector flange, should be an ideal platform for nice even heat.
The book, Manifold Destiny, is famous for vehicular cooking. It's out of print, thus the Amazon price of $90 and $60 used.
This website has some basic car cooking instructions and photos:
http://www.instructables.com/id/Cooking...-with-your-car/
Sherwood