So, I think I got the idea now, though I feel it's a bit spooky. I found one of my electronics books. I remembered that I've learned about realization of non-linear circuits with simple opamps. And that's it, I found it!

Well actually I found it at the limiter circuit, which is made for DC operation, so this may be a serious difficulty. But I'm working on it. The other option is of course the digitalway. Get a DSP with an ADC on its input, set sampling rate around 15-20kHz (maybe even less), and just simply pick out the low amplitude frequencies, and make a conversion back. Just plain and simple.
I don't know it it could work well. Maybe I should try making it.
But first I'll go to the guitar shop, and buy a NEW, guitar cable. Because after some more testing I found out, that I get radio chatter EVEN when the input to the wah is shorted (pickup potmeter at 0 Ohms), and even that stupid high amplitude noise comes in at times. Next thing I'll try now is to remove my cell phone from my room till I play.
Be back with results.