Dave, the problem with votices is only when they slap against something like another rotor or a wing. At the 100kts speed chosen there will be considerable noise/vibration as intermeshing retreating blade sets up vortex for advancing blade to hit. Hence my thoughts on feathered retreating intermeshers.
I haven't looked into it in great detail, but Chinook sounds to me like rear/higher rotor wake impinges front/lower rotor. Has anyone found that tandems get quieter as they go faster?
BTW another concern with interleaving over intermeshing (or even conventional) is the required heli rotor span for same disk loading - the V22 problem...
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"Could the Magnus wing effect ever be applied to rotorcraft"
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I imagine it only works well at a narrow speed range. Too slow and high disk (line) loading is a problem. Too fast and the effective aerofoil shape becomes inefficient.
Besides, i prefer a Flymo to a Briggs&Stratton...
Mart