Comment on Slaying a greenhouse dragon. Part III: discussion by John O'Sullivan

Al, if my ‘ignorance’ is in question then yours can be in no doubt. Just two words utterly destroy your bogus thought experiment: Poynting’s law.
To elaborate, when calculating any transport of electro-magnetic energy (radiation) all such flow is to be determined by field vector calculus (i.e. Poynting’s vector (see Poynting’s theorem) which is an energy conservation law: see John David Jackson (1998). Classical electrodynamics (Third ed.). New York: Wiley. ISBN 047130932X. http://worldcat.org/isbn/047130932X).
Vectoring laws specifically require that when the direction of such energy flow is equal and in opposite motion ( as per your “long room with two flat black walls, each having a certain temperature, T1 and T2, of about the same level” ) all such flows must be summed to zero. This is a law of science and not an option! Thus, just like conduction and convection radiation can have no BACK FLOW.