I dont know about any software for use with UPS's, but if you have a G4 released after Jan of this year it will have built in Fault talarence..
here is the blurb from Apple...
Fault Insurance
...... Mac OS X Server watches over your running programs, and if one stops functioning, the operating system will shut it down and restart that service automatically. That’s not all. In the unlikely event that Mac OS X Server itself is the cause of a system-wide freeze, Mac hardware will restart the entire machine and all of your software.
I'd assume it would reboot after a power out.. OS9 can now...

Why not yank the power out, and see what hapens when you plug it back in??
