...Guess it could get stuck ON if your Batteries died while it was in the on position.
True but I believe there is a low battery warning and don't think that the system, as designed, has much chance of failing open. On the other hand, I wouldn't bet against it failing open once you get X10 controlling it!
I'm beginning to believe to make this project work you would need at least to see the controlling circuit to build your own or hack the original one. If I ended up needing to buy the controller, I would most likely end up deciding the sprinkler control was then to be a non-X10 automated item. i.e. This just might be better done without X10.
That said, I still am curious as to how the circuit works.