Thanks for your thoughts, mackerintel.
I hope I do not get on anyone's nerves here, otherwise please tell me or stop reading :-)
Oone way out of this fixture without risking filesystem corruption could be a separate 8MB partition, or even better, a USB stick. I have in mind something similar to the swap partition under linux, where a signature allows to quite reliably identify the partition/disk.
The approach would be for the kext to access its partition in a similar fashion as AppleFlashNVRAM accesses its NVRAM chip. This means that NVRAM data is essentially kept in DRAM until either it changes, the machine shuts down, or there is a kernel panic.