bbowers:
I figured it wouldn't be too difficult... I was wondering, instead of locating the arduino in the basement if I could replace one of the 12 position panels with it? That essentially has the same connections I would be adding in the basement. I'd obviously need to provide more details once I move in of course, but do you have any suggestions on monitoring whether or not a light is on? I guess you'd have to do something with the line voltage to see if the solenoid is providing the load power or not, I dunno.
Yes it should be possible to interface the arduino and it's new relays near or in the remote control panel. However without a diagram of the remote control panel you may have difficulty figuring out what is what with the existing wiring and how to wire your new relays into the system. Having actual feedback signals to the arduino that specific lighting loads are actually being powered or not, or harder yet, if they are actually drawing any current (missing or burned out lamp) is a pretty complex and expensive proposition, not impossible but more complex and more expensive. That information would be only available at the main control panel in the basement and of course would mean you are also having to interface into the 120vac side of the lighting circuits, not just the 24 volt control system side. Note that your existing system does not tell one if a specific circuit is presently on or off, but rather just gives you the ability to specifically turn a circuit on or off.
Good luck;
Lefty