X10 is probably a bit too slow for cue lights - at times there can be a good 3 second lag on it before the light will come on.
Most theatres have XLR patching to pretty much everywhere. You can use a switch, power supply and small lightbulb to make a really basic cue light system - just wire one side of the switch to pin 2 on an XLR, pin 3 to the negative side of the power supply. Then join the possitive to the other side of the switch. At the other end, join pin 2 to the + on the bulb, 3 to the -'ve. Bobs your uncle.