You don't need to flash the ROM to do htis, and for the most part without buying additional modern hardware, you can't really flash or replace the ROMs on these machines. (it's probably possible to build a replacement ROM but on its own that won't do what you need so I'm gonna call it "out of scope" here.)
In earlier "beige" eras one of the tricks was doing something extremely similar to what you're imagining though, a tool called "Wish I Were" will intercept calls to check on what the computer calls the Gestalt ID, which is just an ID number. You can use WishIWere to trick the mac os 8 installer into running on an '030.
Anyway, in the 9 era: There was a tool back in the day that would do this, called Mac OS 9 Helper. You'd get 9.1 installed and set up, then stage the 9.2.1 and 9.2.2 updates (maybe a 9.2.0 update as well?) and go through the upgrade process one bit at a time.
At the time, people were unhappy with Apple for withholding 9.2.2 from pre-G3 Macs, but in my experience it's not meaningfully better to merit it for its own sake -- the main improvements were for a handful of somewhat niche scenarios at the time, and the remaining one was, I think, an improvement to ipod support. Most of the things 9.2.2 improved upon were really things that worked better on OS X anyway.
All that said:
Do this at your own risk but it looks like someone put all the stuff together and tossed it up on The File Locker
https://archive.org/details/cdrom_OS_9_Helper
It may also still be up on
https://eshop.macsales.com/OSXCenter/OS9Helper/Framework.cfm?topic=32
