Sometimes I feel really silly. Finally having found redfrik's home page (HOW did I miss it?) at https://fredrikolofsson.com I did some reading and found https://github.com/lfnoise/sapf @neauoire i wonder if this is familiar to you?

Phew. Someone asked me if I could make a midi enabled version of billow. Well, it works. Has two modes. Normal, 6 note (or normal play) control which works with long tapes and a midi mode which works with the 5 volt input (phone charger will do) and tape loops. Only tape loops in the pico controlled mode.