After wrestling with very simple single transistor VCAs with not much success, I arrived at the idea of constructing a low pass gate instead for V2 of the synth. Having worked in the past with DIY vactrols, and then graduated onto transistors as a more elegant solution to the issue of adding voltage control to circuits that rely on the manipulation of resistances for control parameters (ie, feedback paths in opamps), swapping the resistors in a sallen-key filter for BJTs was an obvious leap.

This circuit worked well, although took a lot of refining to get it exactly where I want it.