From what I've seen online you need a Mac OS ROM version 9.5.1 or higher to boot the MDDs. Some people also had success booting the first classic-only iMacs using a 9.8.1 ROM that Apple used on a specific MacTest Pro disc (MacTest Pro for Power Mac G4 (March 2003) Version 7.8.1). My late 2002 iBook came with 9.7.1 on the Classic image. Also, if you grab "Netboot for OS 9" from Apple, you can extract 9.2.2 images (Applications and System Folders) from it (using Pacifist) in English, French, German and Japanese that have ROM 9.6.1.
In theory if you either install 9.2.2 via OS X or TDM and then replace the ROM file, it should work. I'm guessing the later the ROM the better, as they should all contain the extra boot and other fixes that the previous versions had, but I can't say that for sure as I don't have any of the "right" machines to test this on. The latest ROM revision I could find mention of online was 10.2.1.
In theory if you either install 9.2.2 via OS X or TDM and then replace the ROM file, it should work. I'm guessing the later the ROM the better, as they should all contain the extra boot and other fixes that the previous versions had, but I can't say that for sure as I don't have any of the "right" machines to test this on. The latest ROM revision I could find mention of online was 10.2.1.