Several of those which had and could dual boot OS X and OS 9.2, required OS X to be installed and present, first and before OS 9 could be used as a secondary boot system. The firmware versions played a large role in this and likely there also were in-chip changes beyond firmware flashing on their logicboards.
I spent some time several years ago trying to discover the problem someone out-of-country had in attempting to restore funcational OS 9.2 booting to a DP PowerMac of this vintage, and the answer came back to the actual need to have OS X (in system software package, part of the computer's included kit) installed first. And if the correct drivers were not loaded at the time, the later OS 9 system in the software kit, would not work. And the OS 9.2 could not be installed by itself.
And the detailed facts of the matter were a hard-sell to those persons afar who did not choose to believe them. Finally they posted their question in these Discussions when their computers were fairly new. And I replied to the same questions in the forum. There was controversy even then as not all believed the facts.
All that, yet you could put an OS 9.2.2 into one of these fringe machines if you did not use their install method. Even a 'classic 9' from an iBook G4 (mid-2005) 12" 1.33GHz software packet can be used to boot up an original OS9-only boot computer of an older vintage. The software would have to be extracted in OS X with a utility such as Pacifist, and the files modified very little, put on a USB drive, and dragged in. However some of the correct files for certain graphic cards or other items would be lacking unless you happened to have one of the last retail OS 9.2.2 full install discs that were not limited to specific versions of hardware. And even just these discs won't work on the PowerMac that requires OS X installed first, because OS X controls the installation. If you get past the need for the 'installer' then the actual system will work; or after the OSX prepared the original setup, it could be removed.
To think I had one & gave it away with a vintage powermac
and extras, to someone who had no clue why it was there.
Good luck & happy computing!