You are correct in what you are saying reference to the EGT, but the the bores and head do get warmer with a very lean mixture. This is almost going into a philosopical argument here, but the siting of CHT and EGT probes is generally very poor in a/c engines and often the results you get don't truly represent what is happening inside the bores.
Octane tests and knock values are usually done on a specific engine design. Automotive octane ratings are determined in a special single-cylinder engine with a variable compression ratio (CR 4:1 to 18:1) known as a Cooperative Fuels Research ( CFR ) engine. I only put this in as most research of this type has been done on a very different engine design to the ones we are talking about, so there may be differences.
When the mixture is leaned to a great extent, the speed of the flame front reduces alot and in some cases it can slow down enough for there to still be a flame present when the exhaust valve opens. Cue a dirty great backfire. The heating of the cylinder walls is greater since the energy transfer (chemical to mechanical) is reduced whereas the transfer to heat energy continues unabated.
We all agree that energy can be neither created or destroyed, when a charge goes into the bore, we have a finite amount of energy. This is all converted. Since the mechanical energy is reduced, the heat energy increases therefore we get more heat transfered.
If the probes were arranged across the engine, then we would see hotspots, but since we usually don't have this apart from in a lab we can only base our judgement on what we see.
For a real simple explanation, when we have excess oxygen any burn is hotter. For a lean mixture we have more oxygen, cue hotter temps.
This is real research stuff and goes into a level of complication that is unnecessary here for flying, but it does make for interesting discussions. (If you are as sad as me that is!)
The method for cooling an engine by dumping whacking great gobs of fuel in has been used many times. I have read numerous books written by WW2 pilots after getting hit in the cooling system. Keep pumping the fuel in and you may get home. Not good for efficency however. What caused the detonation in your friends machine? Was ant damage done?