Assuming you mean in the opamp or transistor feedback path…
My recollection of the Feedback Control Systems course I took in college is a bit rusty, but I’m pretty sure filter feedback operates on a timescale that makes it difficult to make interesting. If anything, it might make the most sense in a semi-modular, noise-hacking environment.
In a delay, the feedback path is recorded back into the buffer to be played again later. In order to hear the fed-back sound the next pass, the buffer has to be sufficiently long – at least one wavelength of the shortest period you can hear (tens of microseconds), but in practice, usually a few orders of magnitude larger than that (tens or hundreds of milliseconds).
In a filter, the feedback path is part of what defines the cutoff frequency and resonance, and the signal is “recorded back into the buffer” at the rate of electrons (speed of light). Any component you add to the feedback path, resistors, capacitors, etc, will change the frequency response, possibly throwing the filter into instability (oscillation), or unexpected (undesired?) output…
Anything beyond a passive electrical component, like a whole other module, seems like it would wind up turning the “filter” into either just an amplifier, or some kind of unpredictable EQ.
On topic, I’m likely going to give the Erica Polivoks DIY VCF a try; seems slightly more tame than the Harvestman, but from what I’ve heard in youtube videos, it sounds nice… Anyone else given this one a spin?