storing data in anywhere on the hdd using low level methods is dangerous, as explained earlier with the controller sample. if your commands to the hardware and os x's commands to the hardware get mixed up badly, you might end up overwriting random blocks on the hard drive. that, of course, will be unacceptable for most users.
i'm toying around with dumping the nvram into a file using a mac os x shutdown script. it gets dumped late enough to capture stuff like "efi-boot-device-data", however the SystemAudioVolume set by appleHdaController is lost.
for the nvram var=value command to work, you dont need a fully working nvram implementation. a dummy one that just reports "EFI_NOT_FOUND" every time will work just fine for that.
so i'm still waiting for a test case which needs persistence across reboots ^^