Cheapest variable pitch done supporting autorotation landings upon powerloss instead of falling dead when no power?
The BEMF from the wind blowing on the electric motors (which are the same as turbine generators) could power whatever emergency processor handles autorotation, or a separate battery/capacitor could.
This would be for if the main battery connection fails, or is shorted out, or overheats, or unexpected EM interference combined with sudden weather changes and crappy RTH-on-signal-loss AI prevent RTH.
Agree, you're describing a Heli... although even with a collective pitch rotor mast, I haven't seen a smaller "common" RC size Heli have an auto-rotation clutch in the drive train. It will spin down, easily seen when you land... takes a minute or two to stop main rotor after powering off... but still has friction on the main shaft.
To autorotate requires altitude, rotor inertia... and a Pilot that pitches rotors to greatly increase mast speed and when near contact flairs the rotors to create lift & slow craft. If performed too early, the rotors loose inertial & speed and she drops out of the sky.
So to truly auto-rotate the craft would need to be higher than probably flying it, and RC Pilot would need to perform the auto-rotation procedure (2 parts: inertia & flair) which might be challenging since you wouldn't know your mast speed or probably flair incorrectly near ground. The fear of battery loss... normally when a Heli battery becomes low, the performance drops to the point that you're coming down under dying battery.
To make this an "electronic package" automating the process would add additional weight. Unless you're suggesting having a backup battery to spin the rotor... a good low voltage "toner" connected to battery warns of low battery.
There are probably talented RC Heli Pilots, 3D flyers that may perform this... it's well beyond my skill set. Just flying a Heli... even the new GPS models is much more challenging than a stable drone.
The benefit of quad is stability and simplistic control that's led to impressive payloads. Until recently, there hasn't been much improvement in Heli's... they're no where near as stable as GPS Drone.
Heli's were long ahead of Drones... and built on a heli frame, not even remotely similar to quads. Scale Heli's are beautiful, some so detailed are simply amazing.
Good luck on your engineering venture... it'd be breaking new ground on some components.
A good parachute system would probably be more reliable in the end.