RVDT, i thought about the phase shift torque measurement technique - i've never seen it in use, so can't comment about it's effectiveness over the design life of an aircraft powertrain. Electric drive is also an area of interest to me, the larger contruction equipment uses it, and i've whiled away many an hour discussing heli applications with Dave Jackson. There are several promising contenders for direct fuel to electricity conversion, so i wouldn't rule out the electric helicopter. My best guess is i'll be making my move into the heli industry at about the same time.
Diesel torsional resonance is not such an issue now as it might once have been. Finite element analysis techniques allow accurate predictions of powertrain vibration modes, allowing design out on the "drawing board". One of the more obvious things highlighted is the need to put the camshaft drive near the flywheel, for accurate timing. Various solutions can be examined to keep crankshaft modes away from drivetrain, and actually this is why the old LandRover TD5 was a 5 cyl design - the 2N (2x crank speed) inertial of 5 pistons acting on crank balances out the diesel knocks (i've got an interesting internal paper somewhere). There are also emerging laser ignition/combustion techniques which i believe will cure the diesel engine of any disadvantage regarding RPM and vibration.
I am convinced that high boost turbodiesels will be the future of helis in the near future. Basically the turbine handles the airflow for power, while the piston handles the pressures for efficiency. Whether i can get back into the development arena is another question.
Mart