Hi Ray – I just wanted to chime in and say that this would be a great addition that should be easily added to Opensprinkler.
The RF route is extreme overkill for most applications. You probably already know this but most home automation people simply add a relay to the line that runs to the button that opens/closes the garage from inside the garage. There are several examples of people doing this (and remotely controlling the relay with something like a z-wave controlled outlet) – here’s one I found on youtube: http://www.youtube.com/watch?v=W8RtzUgupDI
I’m not an electrical engineer, but it seems like Opensprinkler already has most of what is needed to make this work and this would really get the attention of the home automation market.
Determining whether the door is open or closed could be tricky (or added later) but I think it could be as simple as adding an open/closed loop sensor that you typically find in security systems for < $10. So a little guidance and code could really expand your market.