Nick,
Looks interesting so far, I have a feeling its a bit under designed however. I have some drawings of the DH-4 "helivector" and some notes on the development of it. The rotor head is much more complex and robust then your design, and the chain drive might not be a good idea due to driveline harmonics. Belt drive would act as a sort of dampener. The DH-4 was one of the few personal helicopters that actually flew, and it was powered by one of our MK 55 engines of about 50 horse 100% duty cycle.
-Aaron