Today we had an electricity outage cause by thunderstorm. While UPSes were still alive on both network segments connected via ZyXel PL-100 power line adapter, I tried connecting them into UPS and see what happens. Unfortunately it didn't work. When both adapters were connected to UPSes, Link led on both adapters was off. After power came back up, it changed nothing - no connection through UPSes. When I connected one adapter directly to the wall, and one through an APC-made UPS, Link leds came on, but computers still didn't see each other. As soon as I connected both adapters to the wall, connection was re-established.
The bottom-line: If you need network connection in your house to stay on during power interruptions, use wireless networking instead of power-line adapters.