Hi Mark, congratulations on losing your guitar in a righteous cause. :-)
I still think that ring-modding the guitar against the theremin is the way to go. It sounded so good when we tried it.
But I just had another thought. Take a look at this: http://www.madlab.org/kits/theremin.html
For £13.40 inc. p&p it might be worth a try. It's going to have a tiny pitch field, but for your purposes that's just right, and I guess it'll be nasty old square wave, but hey, it optionally quantises the pitch, which could be fun. :-)