What if........
There are too many "what ifs" in the original question for there to be a safe way of leaving the controls while the rotors are still powered.
What if there turbine decides to run-way up?
What if one of the control servos decides to have a "stroll"
What if there is a fluky gust of wind. 407 too suggests "flight idle, friction locks on. (wait for rpm to run down)" - Blade sailing is far more pronounced at lower rpm
Then there are some more "what ifs" for the shut down scenario.
What if the starter motor fails when you are in such a remote location.
What if there is a fire on start up?
If only life was simple.........
407 too's second post talks of the difference between legality and practicality. You try explaining that the accident happend "cos you only left the controls unmanned for a few seconds while you did whatever" and see how far it gets you in court.......