the problem with an analogue light control unit is that its output is triac controlled AC, they are merely on/off switches. I suppose the gas control valve as used on a boiler could be utilised as that is solenoid driven, but i doubt they'd last very long.

I stil think a radio-control model servo would be a good bet for driving the stopcock and simpler to implement than a stepper motor control...

i think your first experiment is to obtain a suitable valve and find out just how much torque is required to operate it... then we could work out necessary accelerations etc and see what is the best driving technology. The rest, getting analogue signal, frequency splitting it, and converting to a control signal is a seperate issue and maybe an existing analogue light contol could be adapted.

(btw I designed and built an analogue light control chaser for a mobile disco I used to run in the 70's.. it was quite sophisticated for the time, with an 8-channel frequency analyser, each channel controlling either intensity or strobe rate, and a seperate beat follower channel.)