Graviman and Dave, may I as an illiterate observe that during BAH's BV234LR introduction we were led to believe by Boeing's TP that a stretched, 4 bladed version was actually built and tested by Boeing. It was supposedly much smoother and faster than our Short Fat Ugly version; however the main customer was the US army who didn't want or need it. I don't know whether this is true or not; if true what happened to the bits? Clearly Boeing were already aware of the problems you mention.
Yes the Chinook has considerable downwash to the fuselage; in flight you could see the passengers vibrating up and down in their seats beside the "airline style" fuselage interior trim because they sat on a "vibration absorbing" floor. So too the external fuel tanks and the cockpit seats. We loved to fly it but nobody wanted to travel in the back...
I don't accept that only the Chinook vibrates badly; ALL of them (ancient or modern) have a critical vibration phase. Eurocopter pilots say that their aircraft vibrate less than most due to the materials employed but it is an inherent problem with any rotorcraft.
Perhaps your suggestion has merit and could be further examined.