It might well be possible to fit the Envelope Filter to a Wah Wah shell and drive the 'fixed' wah control from the foot treadle. The Envelope Filter is set up to sweep the wah frequency from around 300Hz to around 2000Hz with the manual knob going from 100K to zero (plus the 10K series resistor). Most pots designed for Wah use would be able to provide this resistance change and the standard value for them is 100K. As far as I can see you would have to wire the Wah pot to be a variable resistor (i.e. not wired as a divider as they are in the classic Wah Wah circuit) that gets smaller in value as you push the toe of the treadle down.
Another method would be to wire the pedal pot as a potential divider and use the wiper, via a resistor, to drive the base of another NPN transistor. The collector would need to got to +9 and the emitter to the switch. It would probably be a good idea to add a 0.001uF cap from emitter to ground and you would probably need a bias resistor from base to ground.
The problem I have with the manual/fixed control on the Envelope Filter as it stands is that it does not provide the change in frequency that I expect over the rotation of the pot. A log law pot is provided with the kit which means that the 'fixed' frequency hardly changes at all between fully counter clockwise and half way. I suspect you would actually get a better sweep if you fitted the Envelope Filter to a Wah Wah housing with a true Wah specification pot (the ICAR spec Wah pot is essentially a linear law pot over its active rotation with large neutral areas on either end outside its normal rotation range).
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