Thanks FlatTop, I actually like the Hirondelle idea. I went to the LBS today to measure threads and spaces on 6-speed hubs (you should have seen the blank on his face when I explained the project), and I think there's enough space for 2 BMX freewheels on a screwed-on adapter there. I'll see if we have 35x1 cast and dies at the workshop on Monday and I'll start machining it. I also plan on welding an adapter plate onto the innermost freewheel so I can attach a chainwheel on it. That way I can get 44/34 for my backward granny gear, and 44/13 for my forward speedy gear. I just hope that won't be too much torque on the hub thread

Anyhow, if this project goes somewhere, I'll keep you posted.