Coalport wrote: [i]The advantage of the smaller theremins, like the Etherwave and E'Pro, is that they can be played seated which frees the feet for pedal control. I think most people would agree this is essential for effective multiple-voice playing.[/i]
I'm not most people. :-) I can manage two hands and one pedal if I don't try to do too much at once, and I certainly don't have the coordination for both hands and feet at the same time - my control becomes ineffective. :-( Mostly I try to leave my pedals at fixed settings throughout a piece. (With a couple of exceptions - Butterflies Of Vertigo, Dance Of The Flower Pot Men.)
What I have experimented with is doing something similar to what you describe in post production, copying a single recording onto several tracks and processing each copy differently, and I would certainly like to do similar things in real time. (Articulator, Kraken, The Housefly's Lament Remix.)
CV appeals to me more than MIDI - somehow it seems more in keeping with an instrument that generates continuously variable pitches, and I'd like to be able to draw on my experience of dataflow programming too. (Many, many years ago I developed a 3D graphical notation for the programming language Forth combining data flow and control flow paradigms - StackFlow (http://www.taygeta.com/forth_intro/stackflo.html).(*)) The ability to do maths with control voltages is integral to this, and Fred's CV Shift+Stretch is the basis of a useful component. (And, used by itself to drive a second voice, an effect I have not encountered before; a pitch shift which shifts by a variable amount depending on the pitch!)
The other obvious component would be a CV delay - in combination with the CV S+S makes a difference engine - I already do something similar in the audio domain with delay and ring-mod. (Iron Sun, Gently Drowning, Point Of Collapse.)
(*) The third example on the page about the Forth Interpreter shows how the CV Shift+Stretch can convert Celsius to Fahrenheit.(!)