I asked about this here as well, and was met with a STONEY SILENCE, even though I am SURE that someone at Microsoft knows about this one. It seems to apply to the system disk from the bootsector onwards.
Please be aware that even writing a Kernel Mode driver, will not work, if you intend to use ZwCreateFile/ZwOpenFile or something like that. The writes to the system disk are still blocked even at this level. So much for having control of the machine in a KMD.
Again there seems to be no documentation on this issue.
Is the function properly enabled when the KMD or application is signed for example
Note that in some cases this kind of autocratic nonsense by Microsoft causes more trouble than it might solve......