What about incorporating an AC voltage monitoring device that simply plugs into a wall outlet? I think they have those available that activate an alarm when there's a power failure, so instead of that, wire in a relay to activate a continuous duty 12 volt solenoid valve to allow O2 flow when the lights go out. Use a deep cycle battery or two as a power source?
Charge the batts periodically and they should operate the solenoid for a good while, depending on the current draw?
You could just use a small AC powered SPDT relay plugged into an outlet that would use minuscule amounts of energy vs the AC solenoid and run your emergency battery power thru that. Small DC coils can get down pretty low on the amps (1/4 or so on 12VDC), so that wont eat a battery up to quick.
