Dick,
The picture of you baffel setup on your twin clearly illustrates the method of keeping the airflow over and around the engine cylinder. Andy Lennon also stressed this consideration in his book about model airplane desing in the section about cowls. Another point he made is one regarding allowing for an area for the heated air to expand before you send it back out of the aircraft, he mentioned the possibility of using this thermal expansion to creat thrust. But as you said is we need to get the exit flow to work without adding excessive drag, wouldn't a exit at the rear of the airframe serve this purpose?
I am concerned that the hot air exit can cause drag over the rest of the airfram by causing a seperation bubble to occur near the cooling exit and increase the drag caused by the airflow seperating from the wing.

Then on top of that is another consideration, with a high speed aircraft the volume of air moving through the cooling system becomes a consideration. I have notice that many of the exotic speed designs have very small inlets, my understanding is that limits the volume of air moving through the cooling system. The reason for that is as I understand it much akin to the "windchill" aspect that we hear often in weather reports, where the speed of the air enhances the cooling effect.
But to go even one step further when designing your cooling system, shouldn't we take the type of airframe into consideration when we are venting the hot air out?