Well, this is what I've found...
If you copy the system folder over from a PowerMac G (sawtooth , in this case) to the 7300 and delete the original one, you have system 9.0.2.
I updated it to 9.1 using the update.
I can run 9.1 stuff without actually installing it.
The machine thinks it is a G4, but everything seems to work fine. No problems.
So the moral is: yes, you can use a Powermac (and probably an imac) system resotre CD to put 9.0 on a 7300 PM.
I'm even using firewire. Anyone have any comments?