This is a cropped picture of my G5 displaying a Mac OS 9 boot volume as a valid choice when holding down Option and booting the Mac.

Clicking on the OS 9 volume and then the arrow button causes the G5 to read a file from the external hard disk (probably the Mac OS ROM file), and then draw a grey screen for a split second before returning to the screen where you select a boot volume.

It would appear that a number of technical requirements can be met to set up the potential to boot Mac OS 9 on a G5. We can reach a point where the Mac OS ROM file is read into memory and execution of the file is started.

In an OS X booting situation, a file named "BootX" is loaded. This file is the same format as Mac OS ROM but contains only the minimum amount of code to bootstrap OS X. BootX contains the grey Apple, just as Mac OS ROM contains the Happy Mac.

Unfortunately, we don't get to the point of where we see the Happy Mac - or perhaps it is drawn so quickly and then removed due to some other problem.

I have not tried any of this on the MDD FW800 G4 yet but I plan to very soon. Progress on the MDD may lead to progress, or at least further explanation on the G5. Actual booting is probably impossible, but perhaps some interesting information can be learned.