With regard to tracker organs vs. electric keyboards [000121 MMD],
the technology certainly exists to have pulse proportional control over
the air valves, though the cost might be a bit high.  Having spent some
time in a repair shop for model airplane radio control systems, and
having played with pulse power controls for model railroads, I can say
with some certainty that proportion control can be applied to most any
mechanical device which is normally switched on and off.  By the way,
a figure-9 with a model airplane doesn't do the parts any good at all!

Jim Cook