Zvex: Interesting FX... The "Probe" series that he runs has been a source of mystery for many moons - until now! Thanks to the Freestompboxes forum members (~arph and others!) who disemboweled their own probe controllers and donated them to the DIY world :-)
Here's a demo video if you've never seen one in action before:
Here is the schematic of the actual "Probe" controller, you can see that it is a series of inverters driving a simple LED driver - this LED driver is used to control an optocoupler (Also called a vactrol) which is basically a LED stuck to an LDR wrapped in black tubing. The LDR section is then used to control a parameter of an FX unit...
[Note - missing capacitor value at the base of the second transistor - 22nF]
All you need to do to build a Zvex Wah-Probe work-a-like would be to build up a Probe controller using the above schematic and connect it's control vactrol LDR in the position specified in the Wah schematic below!
There you have it, a proximity controlled wah-wah pedal - absolutely useless for that "cocked-wah" sound ;-)