I think the role that would be taken by fixed wing GA has long been taken over by helicopters, in the military.
Austers similar to mine were used by a few countries armies to do things like artillery spotting. The British used to fly them out of improvised airstrips using a ‘low observability’ flight pattern. On takeoff they’d get to about treetop height, and fly like that until getting near the target where the pilot would zoom climb and send instructions to the artillery and drop back to treetop height. On the second trip around, they would look to see where the artillery hit, and send corrections. Landing was done by flying along at treetop height (again, to make it not so easy for the enemy to see where the improvised airstrip was) at 35 knots, and drop it into the airstrip – this must have been pretty nervewracking as flying level at that speed means you’re basically in the 3-point attitude and can barely see anything forwards (although it’d be easier with the narrower Gipsy engines than a Lycoming). Not to mention there’s not much margin over stall speed. There was an article a while back about how this worked in one of the Auster club magazines.
They can operate high enough to avoid munitions,
I rember at Saigon and Salalah having to climb to 12,000 ft overhead the airfield to avoid small arms fire!
Perhaps I’m putting words into your mouth, if so, apologies.
I’ve also thought about how we could use our aircraft to help the people affected by this conflict. Unfortunately unless your private aircraft is pretty big, it will probably be more of a burden than a help. I guess the best thing I can do is send the cash that I would have spent on fuel to people actually helping on the ground.
I googled for a TB20 STC for that, so far without success 
That was my long term career plan; to make enough to buy a TBM.
How to find a GA plane?
Things actively radiated from the aircraft: heat, sound, transponder, DME, VHF. The last three can be easily turned off. Have I missed anything else? Is it possible to detect a mass of powered-up avionics even if not transmitting?
Passive detection: radar, visual. Receiver radio direction finding is possible but very difficult.
Treetop height and below is probably the safest place to be. The mission profile is largely the same as drug-running, and using the same aircraft types too.
The smallest non-trainer operated by the USAF is the King Air, as the C-12. I assume nothing smaller could carry countermeasures and a meaningful payload. Also, they probably aren’t used unless there is air superiority.
Capitaine wrote:
Is it possible to detect a mass of powered-up avionics even if not transmitting?
I’m fairly sure it leaks some electromagnetic radiation. Humanity (civilians) is able to make very sensitive equipment (telescopes) described as “detecting from Europe/America the rear lights of a car in Australia”, so detecting that leakage only a few (dozens of) km away doesn’t seem completely mad. However, I wouldn’t be surprised if there were so much noise that it would be “difficult” to make out the emissions of the avionics of a single plane.
The usual challenge is to get sufficient directional resolution, at the wavelength, and with a sufficiently small receiver antenna assembly.
These things are easier because they normally go for a microwave emitter.
The emissions from non-transmitting GA avionics radios would be miniscule in comparison. I would be amazed if it could be done. But you never know; lots of GA radios push out enough spurious RF to jam the plane’s own GPS (11th and 13th sub-harmonic of GPS L1) 
Capitaine wrote:
Treetop height and below is probably the safest place to be.
The safest place to be is a run down 152 or alike – there simply is no case for the attacker to waste a 100k missile on a 25k airplane.
Peter wrote:
Lots of GA radios push out enough spurious RF to jam the plane’s own GPS
Although that’s not hard to do, given the low power of GPS signals and the distance over which they travelled. An emitter that’s two feet away can put out a minuscule signal at those frequencies and jam them. The L1 received signal is around -125dBm (a bit under half a femtowatt) so it really doesn’t take much at the right frequency to raise the noise floor sufficiently to block it.
By comparison, the Garmin GTR225 radio specs show you start getting a readable COM signal at around -107dBm – so the weakest signal you can successfully read on your COM radio will be about 70 times the power of the GPS L1 signal!
