a bit more digging has found this option (google translate available)
https://www.velovert.com/news/15828/tire-velo-le-petit-coup-de-pouce-malin
Update on this. Got a tire velo for Christmas and did a first ride with it today. It’s bade of the same kind of stuff as a bungee cord, so lots of give, but it also retracts automatically into a dog lead like thing that attaches to your seatpost.
Loads of advantages to this. There’s no way to get the cord caught in a front wheel, which is good, but also there is zero faff in clipping and unclipping the towed rider. After a couple of goes on fireroads my 6 year old and I can both clip and unclip without stopping, I can just reach behind my saddle, grab the hook and place it over his stem.
Super impressed, a solution that’s miles ahead in terms of faff than any of its sprung competitors.