No, You must be hard on the trailer brakes, are you? 4 cylinder engine not sure if you will get much braking effort, but definitely worth a try. I'd say you will have to go the custom build route.
Ford had a truck out a number of years ago (Ford Transcontinental) they were all fitted with Cummins engines that had a very efficient exhaust brake. It was a flat plate that completely closed the exhaust off. It was just after the turbo in the exhaust system. Most of the modern types are butterflies that don't fully close off the pipe, especially after a few hours work. You may need an air supply as they are all pressure controlled, but may be easily converted to electric/hydraulic.
It certainly would be a nice winter project. Let me know how you get on.