I was thinking of a sort of electromagnetic braking disk
They're called electromagnetic clutches; typically, they are mounted on a shaft, and you apply a voltage (12 volts is common) to "lock" it up. They tend to use quite a bit of current, so you would need a hefty transistor or MOSFET interface, like controlling a relay or solenoid (remember the blocking diode).
Potentially, you could control this using PWM - that would be the first thing to try; if that wasn't satisfactory, then maybe some form of PWM to voltage or current controller would work better.
Another option might be to attach the shaft to a DC motor, or maybe a stepper motor, then actuate that as needed; this might be an issue, though, as if the operator let go of the knob, it would start spinning in the opposite direction he was turning it in - not sure if there would be a way around this or not.
Hope this helps...