I have tried:

1. Using the "bless ... -setBoot" command (using Terminal, no errors)
2. Scripting the OS 9 "Startup Disk" control panel ("You can't use that in OS X!" Error)
3. Recording while changing system settings (absolutely no result)

And I just can't get it to change.

For elaboration:

I am trying to change the boot device from OS 10.4 ("Scott's iMac G3:System") to OS 9.2 ("Scott's iMac G3:System Folder") with AppleScript automation.