Hi Graham,
I was in just the same situation as you - I had a period of shift work sprung on me at short notice, so didn't have time to tinker anything up myself. If I had done, it probably would have looked a bit like the chuxaway. An up and a down sensor/switch, plus a timer and motor.
However, the VSB has several advantages in my mind:
1) It works on light, so once you've got the sensitivity right, there's no further messing required
2) The door does not fall to a given position, it merely drops until the tension comes out of the string. That means that if a hen is in the way, it will stop falling.
3) No external wiring or door sensors are required. The door lowers until the string goes slack, or raises until a knot in the string pulls a magnet away from a reed switch (you position the knot to correspond to the door's 'up' position).
How many houses do you have? Also, are they in any way adjacent to each other? It is possible to use pulleys so that two or more doors can be operated on the same motor - just a thought.