Neat!
I too have found that rubber/plastic tubing has to be pretty tight to run a sewing machine. I use Auto vacuum tubing - the only tubing the near by auto shop had that was long enough - which is a bit wider than the usual 3/16" and has much thicker walls than the gas line hose I was originally going to try. It's made from rubber very similar to tire inner tubing. With the thicker walls it seems to take to treadle belts very well, I've had very little trouble with it stretching out.
Though, since it is wider than the usual belt, it does tend to ride ON rather than IN the grove on the hand wheel, but this hasn't been a problem, just an oddity.
I've also thought about wrapping a rubber band around the wheel to keep the belt from slipping. My slippage is on the cabinet's flywheel though, and I haven't found rubber bands that big yet
