@zefrenchtoon Thanks for the pointer!
Btw, I tried to use the Mac mini ROM with Mac OS 9.1 (System and Finder files replaced accordingly), and, it bombs during Start Up screen.
Oh well, it would have been too easy otherwise.
I may try 9.2 and 9.2.1 next, but, I'm not sure I care about these so much, since I'm too happy with Mac OS 9.2.2 to try anything else, to be perfectly honest. But it would be great if people got the mini to boot 9.0.4 and 8.6 somehow.
Or, even earlier than 8.6. New World ROM Macs, with support for the separate Mac OS ROM file, got introduced with Mac OS 8.1 for the Beige PowerMac G3, so we could theoretically go as low as that. To go even lower, I believe we would have to patch up the System file of Mac OS 8.0 and lower with whatever was added to the Mac OS 8.1 System file, much like how we patch System 7.5.5 ~ Mac OS 8 System file so that they support HFS+ just like Mac OS 8.1, which @ELN pulled off in 2020.