Nice work!
I've been brainstorming a similar project for my security system. I'd be interested in more information on how you interfaced with your existing security system.
I don't own the system in my home, so I am trying to keep my interface minimally invasive. My specific unit has a grid of led status indicators to display the armed status and even which specific zone is open. I am contemplating an array of phototransistors to trigger my inputs, but that would get messy fast.