Got a power cord!
The iMac G4 was able to boot to the ? folder, but I heard some buzzing that repeated a couple of times. Connected it up to the Blue Dalmatian, and my fears were confirmed. The hard drive was either corrupt or wiped. There was no system folder. Ugh.
And since it's the USB 2.0 machine, it has no support for OS9. So there goes that idea...
EDIT: I reversed the Target Disk Mode, using the HDD of the G3. I got the G4 to boot 10.4.11, and it's running pretty smoothly. (Typed this edit on it!) Now to reformat/swap the HDD, get a DVD copy of 10.4, and do something else...