How about using onlyonemac's suggestion and to combine the voltage regulator with the computers own PSU: tap some Volts through the voltage regulator to power the clock chip and add a delay circuit to power up the mainboard only a second later. Get the correct time from a time server and run an AppleScript programme to restore other PRAM based setting. In case of the P475 there are also instructions for an electronic circuit available to provide a soft power on (using the keyboard power button).