It's certainly feasible, but IMO not practical, simple or an efficient use of your time... unless you are an electronics geek desperate to learn.
You shouldn't need anything fancy like a Varistor, but you're on the right track. I'm sure with some searching at places like diystompboxes.com you'd find schems for LFO's (low frequency oscillators) and voltage controlled filters that you could mash together. (A wah is just a bandpass filter, BTW.)
If you were me, you'd just go and buy one and mod it for bass if necessary.
I guess what i'm looking for is a "modulated bandpass filter", like what is on the EHX Worm (and not the Frequency Modulator like I thought). The Electro-Harmoni call it a "Neo-Modulated Wah Phaser".
Looking at the video they have on the site, it seems to have a wide variety of tweaks & such.
So i guess Niftydog is right. Id be better off getting the Worm and modding it for bass, if necessary. it would certianly be easier (not to mention less costly, in both time, money & sanity
So I guess that closes this case...i'll stick to building and modding the octave fuzz for now.