Interesting articles SASless & Ian.
There may also be a good case for developing ABC tandems, since the powertrain layout is already there. There would be a weight penalty because the high hinge offset rotors would load up the transmission, and the fuselage would also be loaded in torsion. I imagine the loss of payload would still allow a good payload mass-speed product.
Anyone know how much Chinook powertrain weighs? An initial guestimate would be to double powertrain mass (since engine power also goes up), and see how much payload/fuel capability is eaten up...