you can put a float switch on your main tank to turn off the return pump.
In a perfect world, the CPR overflow will still have siphon if the sump is full and the tank is low. when power returns, the water levels rises in the main tank, and the overflow will continue to siphon the water to the sump.
i have the standard overflow box with the U tubes. I cant keep a siphon going if the tank goes low and the sump goes full. if power returns, my tank may overflow. havent tested it yet, should of done this outside before setting the whole thing up.
i going to either buy the CPR overflow box or somehow mod mines with the aqualifter.