The dampers can be turned with slight finger pressure. And I do mean slight. Johnson did a hell of a job on that, back in the 1970’s. They are silky smooth. The main blower is 3600 cfm continuous air, which has quite a bit of air moving. I don’t know what it looks like inside but I imagine there is already some sort of bypass that lets air through even if all dampers are shut. There’d have to be. Or the main blower motor would be blowing seams out when all the dampers are shut. 
As far as activation there’s not much - inertia is a bigger issue, it’s very easy to turn them but you then have reverse torque to hold them in position as there’s mass. I would imagine I will need to be careful with how I ramp up and ramp down the speed of the steppers to account for the rotational mass still wanting to keep moving…
That is an unknown. I was wanting to rely on the steppers with limit switches on the outside. I imagine I could connect in a potentiometer via a gearing (gear mesh between damper shaft and potentiometer shaft), to give me an absolute position, and feed it through the LTC2495 analog chip. That gives very high resolution readings on voltage dividers - clear down in the nanovolt range (16 bit ADC w/ 256x gain is +/- 9.6 µV over 32,000 steps @ 5v - very high resolution, those chips).
That would give me an absolute position reference that is highly accurate completely outside the context of the stepper motor circuit itself. I’d have real time feedback of precise positioning.