555 timer, big capacitors, maybe an inverter, straight to the power button. Power this with your "mains-ok" line so that any ups fucking with the auto-boot on the motherboard will be taken care of.
Shitty microcontroller + ethernet whatever
Button pushing robot
Prevents void warranties
CD Drive button pusher
Position a second (presumably always-on server) in front of your to-be-powered-on-server, glue a plastic dart onto the cd drive, eject drive to push button