One thought about using a USB volume control mentioned above would be to control an application such as Gary's Regal Chorus
http://www.organexpressions.com/ as a stand alone Reed division using a Mac Mini. The volume control via the USB unit and use the built in 2 channel audio output of the Mac Mini for this division driving a separate amp and spkrs.
Another thought, a while ago I saw on a You Tube video of (I believe) of a UK cathedral organ, that has a slider (control) mounted under a manual on what is normally a thumb piston rail to use as an auxiliary control (in addition to the normal swell shoe) to control the swell shades
if both feet were busy at the time. It would be simple to replace and extend the potentiometer of the USB volume control to a remote slider or a swell pedal (potentiometer).
Last thought -
"Elon Musk has launched a company dedicated to linking human brains with computers, The Wall Street Journal's Rolfe Winkler reported Monday." http://www.businessinsider.com/elon-musk-neuralink-connect-brains-computer-neural-lace-2017-3Just think how much hardware and interfaces we could eliminate with that setup. Scary.
Rgds,
Ed