After working on this project on and off (but mostly off) for several months, I finally completed a prototype in Eurorack form factor (or an approximation of it) for a friend.
It is a pentode vacuum tube wavefolder-VCA-overdrive/distortion (depending on where you place it on the signal chain). The nice thing is that it should work with any dual control pentode tube. No need to chase specific tubes like the sought after Russian subminiature pentodes or pay extra for the $30-a-piece dual triodes. A nice way to put to good use some of those unwanted cheap “by-the-kilo” TV and radio tubes!
I will soon post the details of the build as well as a verified stripboard layout (perhaps also a sound sample from said friend).
EDIT: Extensive write-up and verified stripboard layout here.