Dave, the limitation is that the rotor will likely break off if Nr drops too much. Unless you are throwing lots of would-be-payload mass at the rotor, how is your design any different? Most collectives only go to flat pitch.
If you're refering to the right yaw when the single rotor helicopter lost power, then you include a gyro (laser most likely) so that pedals directly control yaw rate. There is no reason the collective can't be included in that SCAS for Nr control.
What i was really getting at is that current light helicopters do not have a centreing trim mechanism on the cyclic. The fact that most helis aren't designed for hands free flight does not mean that they can't be.
I think many of the control limitations are historical. Pilots are used to the heli being difficult to fly, so this becomes the market expectation. There is no need to fundamentally redesign the machine when some clever control engineering will give you what fixed wing pilots take for granted.