Using air conditioning as additional descent brake

IIRC, the pushbutton transfer box drops the aircon out when 4wd low is selected. Not sure why

Does the AC compressor run constantly when the AC is switched on? It doesn't in my Liana. It could cut out part way down a decent. It will provide a bit of breaking when running but I wouldn't rely on it.

It cuts in and out all the time. It makes as much difference in terms of power for climbing as it does for breaking on a descent. So you do notice when it cuts in.

Well I was on a holiday and I drove down the same descent every day in the morning for six consecutive days, so I had some time to study this question

Operation of the air conditioning system is not affected by the operation of electronic transfer case in my DDiS Jimny made in 2006, and vice versa. In other words, AC continues to work when I engage 4WD-L, and I can turn AC on or off during 4WD-L mode.

Wouldn't the compressor work all the time if the temperature knob is set to the coolest point and the fan blower strength is increased to at least level 3?

If you turn on all the lights and the wipers, then the alternator will also work harder and soak some more engine power.
In addition you could turn the engine off prior to the descent, to run the battery down a bit, before restarting.

_{Note. This is intended as a sarcastic reply and should be read as such.}

Another thing that can help is to decrease weight by for example expelling all the passengers from the vehicle before descending. That would make it much lighter, and by the laws of physics, less mass achieves less acceleration due to the force of gravity.