Is there a working trick to reduce the fogging? One that I know of is opening the light for half hour in a cold airconditioned room (should you have one) before you go diving. I’ve also seen people inserting a small silica gell bag somewhere under the spring. Does anyone have a proven experience with it or perhaps something else?
Leave it open in a bag o’ rice?
Insert the battery in a low humid environment. 
Since cold to warm causes condensation in a humid environment, perhaps try installing the battery when the battery and flashlight are significantly warmer than the ambient temperature?
I think the key is that the air inside the flashlight is as dry as possible or in other words has the dew point lower than the ambient temperature of water (or air) in which it will be eventually used, to prevent condensation.
I wonder if the divers here had a problem with condensation in flashlights and what worked?
Maybe fill it with nitrogen?
I’m sticking with my original suggestion.
Works with waterlogged phones, so…?
Wellp, unlike silica gel, you can eat the rice.
I’m sold.
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That is an excellent point - it won’t mend your phone but it will make you a consolatory meal.
Only dive in warm water…?
Jokes aside, there are anti-fog products you can get to apply to glasses/visors that claim to reduce fogging, I wonder if manufacturers could produce a similar layer similar to the anti reflective coating they usually use…
From a technical perspective, condensation is a result of the temperature of the glass being sufficiently low enough that the air in the head of the light is oversaturated with water so it condenses.
I would have thought the manufacturing conditions in China are fairly warm and humid, so “trap” moisture inside the head. To prevent flogging, you’d want to somehow remove as much water as possible from the air inside the head of the light, AC as you suggested would likely work, due to the dehumidifying effects of AC.
Plastic lenses, with lower thermal conductivity than glass might help too, the inside surface may not be so cold so condensation may be reduced.