How about the low voltage out door yard lights, I think they are 12 volt, I do not think I have seen any 12 volt timers.
If you go with a 12 Volt set up I would also invest in a inverter so you could run small power tools in the coop if needed, probality 400 watt would do, I would assume that if you where doing and heave work you would runn an extenstion from the house or use a generator.