Here's a screwy idea...
Instead of a "normal" VCF, use one that has an insert path...and then drop something like a delay or reverb into that. Doepfer's A-106-1 fits into 14 hp, leaving 4 hp for an effect to screw with the resonance path...HN's FX Aid could work, and there's several others that fit in that remaining space.
The 106-1, fyi, is actually a version of the MS-20's Sallen-Key filter pair...which is definitely is capable of some weird filter interactions. Plus, this idea is pretty cost-effective and fills the 18 hp hole.