Luckless, I don't know if you're saying auto fire as in full automatic fire of the gun or thinking that UAV means that no one is controlling it. Both cases you'd be wrong, a simple servo could actuate the trigger on a mounted AEG, or a relay to the micro-switch in an M249 AEG would answer automatic fire.
UAV simply means "Unmanned", does not mean that its has to be totally autonomous; just that the operator of the vehicle is not on board but simply controlling it from a protected remote location.
Labo, I'd suggest highly against having power cut to the whole vehicle, simply to the weapon alone would be suffice and reduce chance of a "failure" where the vehicle will get caught in terrain. Basically if all power is cut there is the chance that on board fail safes to the receiver not work properly and possibly cause harm to the vehicle. This is why I would suggest the red flashing beacon light; like our red flags, it would signal that the vehicle is down. Vehicle Operators would be held to the same honor system as the rest of us, should their vehicle be hit and the beacon flash to stop firing as the vehicle is "dead".
I think though that the problem is that aside from recon, R/C vehicles would be very hard to implement regulations on in our game. R/C aircraft have the unique ability that they really can be used significantly for recon purposes with out "Entering the battlefield". As long as no weapon is attached to an airvehicle I see no need to be "able" to fire upon it. It would be similar to the Predators today which are rather small in size and fly so high that there are few weapons avail. to hit them. That said one could implement into scenario's AA stations that have such weapons and basically take such a unit "out" of the game and force the operator to remove the craft from theatre and land it. One could even then implement limited "spawns" as replacements for the feild, simulating relocating/take off of another aircraft to the theatre.
Land based becomes hard again though because as Recon units its directly in the line of fire, thus making sense to be able to open fire on the damned thing. It bloody well can't fall from the sky ((which also opens the question of what or WHO it lands on)), but would need an intricate new set of rules.
"The hydrogen economy car from the people who brought you the 'Hindenburg
'" - Glen Foster
Condoms do not guarantee safe sex any more. A friend of mine wore one and was shot by the woman's husband!