........., that doesn't engage until about 2500 rpm. Now THAT would be perfect for the single rotor. The only downside is drag from the freewheeling prop if the engine quit.
How about a simple band-brake (or a disc-brake) to stop the prop " in case".
It will also work as a safety brake when starting and idling. But make the brake handle cross the throttle so you can't accelerate when the brake is engaged.
(At my last job, I burned a clutch, because the (tight-fitting) brake was not all the way open, but the controls are in different locations - brake on the panel, throttle on the floor. A III / Astazou engages at around 30K rpm, clutch was about 13K $ + a whole day cursing and sweating in the jungle ..... other than that it was fun :))
TJ
"Keep the gyro" :)