RE: Theremin cello
I have no information yet on Mr Engels. Doing researh on the web on how the fingerboard might work I found a product made by Doepfer . It is call the R2M -> Ribbon to Midi controller. They used an industrial “Position Sensor” for the controller. Check the web site for more details http://www.doepfer.de , it is very interesting. You can download the user guide for the product for more insight information.
I´ve seen the Doepfer products, they are interesting. When I get the time Ill send you all the internet addresses Ive found (there are many) dealing with ribbon controllers, linear potentiometers, conductive plastics etc. I have had some success in my kitchen myself, creating quite a few linear potentiometers using various strange mixtures of epoxy, paint, graphite powder, carbon powder etc.