I'm slightly puzzled why not use one of the many variable pitch props available? Alternately let the blade feather, with a fixed tab holding a constant AOA, to allow autorotation. Wouldn't involve much machining.
However, autorotation will be at about the same rate as a round parachute with diam equal to rotor. That is one small parachute, and you can't roll on landing to dissipate the energy...