My guess is that the reason the cylinder temperatures are lowered during idle is that there is less coolant(oil and water) moving around the engine at low RPMs to disipate the heat. If the cylinder temps stay high at idle, the oil probably burns, and the engine could need to turn the fans on, which in very cold weather, with slow coolant velocities could freeze the radiator. Checked my 94 Honda DX intake hose, and it has a plugged intake, that looks to be, in about the same place as my 74 Ford's was. The Ford had a bimetal valve that controled a flap to allow warm air from the exhaust manifold to enter the intake plenum. The Hondas and other 4 cylinder vehicles used the same type of system, so checking a wreck from the 70s or 80s should turn up some choice parts. While this may improve your MPG, don't ask me what it will do the engine at idle. Come to think of it, I believe that the valve was, blown shut, by the higher air velocities above idle, and was designed to improve idle performance in cold weather. Back in the daze of Sunoco 260 octane, they used to put a spark plug at the end of the exhaust pipes and detonate waste gas as a show.
The Honda CRX HF and most newer hondas (and other cars) have Exhaust Gas Recirculation which reburns the exhaust **** to the extra gas combusts again instead of dying in the cat. This equals more MPG for Honda, but less for some of the ****ty US manufacturers, go figure.