I've thought about airsoft drone usage quite a bit, ever since I saw a related thread on airsoftmechanics a while ago. I fully support the use of drones, whether they're normal planes or quadcopters. However, ff they can't shoot at infantry, players shouldn't be able to shoot at the aircraft IMO. Maybe just have a capturable SAM site as an area denial for the drone, if it actually has strategic significance. Either that, or you just have to hide in the woods where the drone can't see you. Honestly, I don't think a drone should ever be shot at, because if you do take it down, you have just created the biggest possible aircraft-related safety hazard (having it falling onto somebody).
When you get down to the technical aspect, I think a long-endurance plane would be much more practical than a quadcopter, for the simple reason of operating time. There's a reason that Predator drones are fixed wing aircraft rather than x-copters. Of course, any sort of aerial recon has the potential to be super cool and add a lot to a milsim experience.
As a side note, the airsoftmechanics thread I referred to was actually talking about airsoft air support, in which the aircraft would actually be armed. Now this throws a whole bunch of new aspects to discuss, but I think figuring out the gameplay mechanics of a simple recon drone would be a good start.