I like the idea... why not?

its not designed for one

a BlueOx type in the exhaust should work if there's enough room under the car for it. Would work best with a manual of course. Thought about too.

Jake brake is a specific brand of engine compression release brake. Jakes work by hydraulic action of the injector rocker arm pushing in the master piston, moving engine oil to push a slave cylinder out and open the exhaust valve. That releases the energy put into compressing the air as well as makes the piston pull a vacuum on its way down.

Since our engines don't have any parts under the valve cover that move during the injection cycle, there is no way to correctly time opening the exhaust valve.

If you want an engine brake the best methods are to install an intake throttle or an exhaust brake valve. With a non-locking automatic, an engine/exhaust brake won't work very well.

http://www.pacbrake.com/files/products/high/engine_p80.jpg
http://www.sweethaven02.com/MechTech/EngMech/od162070067.gif
http://www.jakebrake.com/images/g_howitworks_compression.gif