I agree. Some of that 50% I hastedly attributed to the exhaust loss probably is radiant loss from the engine surfaces. I was just trying to work back to make sense of hotater's coolant observation. In my mind at least, the magnitudes assigned to the the mechanical motion (~30% and probably a liittle less) and coolant, somewhere around 20%, make some sense. Since I'm new to these, I can't make a good judgement on radiant loss.
What I was trying to decide, and I believe I have, is the expense of the added plumbing outside the engine room, worth it for the heat gain from the coolant. Now, I would have to say definitely not, unless heat exchanging from the exhaust. At this point, that nut is too hard for me to crack. What I will be doing is to use CHP inside the room to heat vegatable oil, along with the radiant heat giving passively from the engine.