Well, obviously you can't do what Apple is doing, because you're not running UNIX with OS9. I'm not very familiar with AppleScript, but you might be able to write a script that would delay a restart.
why run OS9 on a G4? OSX client and server are both optimized for a G4 and as long as you have 512MB ram it will be nice and snappy on a G4 of any speed really.
To comment on your question.. wouldn't installing 10.3 server let you do everything a mac server can?
Can't do OS X 10.3 quite yet on this mail server. That's why I was curious whether it could be done in Open Firmware. It will be a feature I'll likely use when I move the server (or get a new server) to OS X.