How to write drivers for most things for classic MacOS is reasonably well-documented. It's just that the number of people writing low-level software for these machines is low. I don't think having the OS 9 source code would necessarily achieve much other than making people even more worried.
Yeah, it’s possible to write drivers and patches for classic MacOS, including MacOS 9, and always has been. Apple has been documenting the relevant requirements since 1984. If you have specific questions this is a good place to ask
Decompiled ROMs for the various machines would probably be more along the lines of what you're looking for.
It would be truly nice to be able to fix the bug in Apple's Open Firmware implementation on the X500/X600 (also Gazelle) machines, which causes traversing PCI busses properly to fail.