Can anything interesting be done with Kidde's interconnect wires?
First, don't use the wiring for anything else. It's a life safety system. Don't mess with things that are this critical.

Second, the signaling is simple: voltage on the line means one of the detectors has detected something.

To address your ideas:


Anything but the cheapest detectors has an indicator light on the front that tells you which one has been tripped. For example, my First Alert smoke detectors have a solid red light on when it detects smoke. The el-cheapo detectors installed by the original builder did not have such and indicator light.
Again, anything but the cheapest detectors has an indicator light on the front that tells you the state of the battery. On my detectors, the green light tells you the state of the battery and the line power with various blinks.
Don't mess with your life safety systems.