Originally Posted by
Kapusta
I don't think that a power meter on the rear hub and cranks is going to fully answer the question of efficiency and power delivery. (thought it will answer some of the more specific questions, here). It will let you know the efficiency between the pedals and the wheel, but it does not tell you what is happening on the rider's side of the pedals.
It would measure power loss from the pedals through the bike frame and drivetrain. At the pedals you are measuring the rider's torque and cadence to give their power output. I guess you are asking if the frame is flexing enough to directly affect the rider's pedalling efficiency and power output? I suppose it could in the extreme, but that really is going to be very subjective. My own power limitation comes from my engine! Pedal forces are relatively low.