Mostly you'd be interested in the /tbxi15.src/Bootscript.
Actually, I found another way to edit the bootscript, as follows. This assumes you have a PPC Mac with Mac OS X, and a 9.2.2 installation on the hard disk for use with the Classic Environment:
1. Boot up any OS X system you may have installed on your PPC Mac.
2. Find the Mac OS ROM in the OS 9 System Folder using the OS X Finder.
3. Ctrl+Click on the ROM -> Open with -> Other...
4. Find TextEdit via the "find" dialog that pops up, and click Open. The first couple of pages in the resulting TextEdit window will be the bootscript.
I did this under OS X v10.4.11. I encourage others to try, and let me know whether this method works for them, in terms of enabling editing of the bootscript.
QUICK EDIT: I used this method to manually incorporate the changes @darthnVader made to his bootscript into the Mac OS ROM that is already on my iBook G4's hard disk, and now the error I get in OpenFirmware is "no active package", which I assume has to do with the portion of the modified bootscript that ascertains the CPU type (at least I think that's what it does...?). How might I fix this? I have made sure I typed it all correctly.