It is not a good idea to try kludging up I2C.
Whatever the voltage it is pulled up to,
it will be vulnerable to noise.
When a slave pulls the signal down, there's a
low-z path to ground. When floating high,
it is high-z and thus sensitive to noise.

This kind of thing has all been done before,
nearly a dozen times, for factory automation.
They're called field buses. Each has pros and cons.
Check put SDS Devicenet (based on CANbus)
or Profibus (based on high-speed RS485)
to get an idea of what's needed to do the job!

Unless you want to spend millions (really!) developing
protocols and hardware, I'd suggest a low speed RS485 network.

All the home security systems I've seen are pretty dumb,
in that the only output is an alarm.
I thought about designing a system where a central
low-power PC could log events like PIR sensors
going off in the back garden at 3 a.m.
Even if they don't break in, its worth knowing
when someone is casing your joint!