Dafydd,
If you lose one magneto, it's advisable to keep the mixture full rich - tho that is not commonly taught, except that if the engine runs rough, or the EGT rises noticeably, most pilots go to full rich automatically.
Hang on.....
Here is how I thought things worked:
The fuel is pumped in, compressed and the spark makes the BANG.
If there the air/fuel mixture is good, the temps are good.
If the fuel is too low, the cyl.... pistons get hot, the valves, etc and you get damage.
That is because there is too much oxegene that wants to keep things happening. Yeah, ok, badly explained, but I hope that is good enough for this.
However: If there is TOO MUCH fuel, it isn't burnt and so things run cooler. Also there is more "smoke" put out because of the un-bunred fuel.
So if - as you said - a mag is lost, all the fuel wouldn't be burned and so the EGT would go down. Right?