Dave, it seems to me that for any of this to work, you have to get the weight off of the rotors during high speed cruise. In a conventional helo, the rotor has the provide both forward thrust and lift so at least theoretically, the rotor speed has to be maintained.
In a compound helocopter, you have some other device providing the thrust. So likewise it seems the only way you can slow the rotor down in cruise flight, is to have some other surface providing the lift (at least a major part of it). This seems the only way to slow the rotor down and still maintain a decent low drag AOA for the rotor blades.
I'm often amazed by what was accomplished with the Fairey Rotodyne for its time. I know it used autogyro principles, but this is in the end, a variable speed rotor.
Fairey Rotodyne