Dave, having given the design of servo-flaps a small amount of thought:
I would say that the response lag will depend on the blade inertia about the pitch axis, the distance of flap behind this axis, and the linkage gearing for flap_pitch/blade_pitch. The other way to think about it is that the servo-flap/blade/linkage system will have a natural resonance, albeit likely to be well damped. If the servoflap/blade/linkage system natural frequency is sufficiently high above the rotor rotation frequency then the response will not be an issue. It should be possible to determine how much swash plate lead is required to correct for the servo-flap lag.
I think this system could be made to work with modern rigid rotors. I suspect that it works better with softer rotors because the servo-flap has an immediate effect on the tip, without the inertia of the rest of the blade causing lag. The vibration suppresion systems should also benefit a reduction in hydraulic dependancy too.
Now if this tip control system was used in combination with root pitch links...
