Random,
An interesting question/scenario. You are sort of on the right track as far as power supply to the Centre System is concerned. Indeed in normal ops the Centre System is powered from the Left Main AC and DC buses respectively.
However during triple channel autoland ops the Standby System is the independent power source for the Centre System and supplies power to the Centre DC Bus directly from the Hot Battery Bus (via the battery charger) and the Centre AC Bus is supplied from the Standby Inverter.
However, as for what is supplied/powered from the Centre System, I can only suggest getting your hands on a maintenance manual/avionic engineer. Hope this helps.
McHale.
