This is an honest question, no provocation intended: is the kernel source available for MacOS 9.x and earlier?
I am not sure about OS 9 (I try not to use it) but I do know that System 7.5 is available for free. Mac OS X is completely open at the kernel level. However, unless you want to kill your product, I would never touch the code of kernel from the old "Classic" Mac OS. I am a UNIX fan and not a fan of Apple's past OS design. They got it right with OS X.