As cortez said, you really should not worry. The Z actually has 2 NAND chips, one is the 128MB flash (/dev/mtd1) we use for OS and the other is a 8MB (/dev/mtd0) chip that is only used by the diagnostic menu and the original bootloader. As long as you don't touch the contents of /dev/mtd0, your Z will be practically unbrickable. AFAIK the 8MB chip is read only so i don't think that you can write or delete something. And again, as cortez said, use the FN + D + M. to access the diagnostic menu, from there, you can do a flash restore which will reset your Z to factory defaults.
And i'll say it again, it is really hard to brick your Z via software methods. If you really want to brick your Z, use coffee as cortez did.