I just patched the serial and pcbsn in my custom u-boot binary, replacing some of my hex characters with ascii zeroes. It does not boot. They must contain a check-digit or something. And of course it WROTE the idme vars, so it is bricked until I patch some real values in my u-boot and let if write those to mmc...
Fun, huh? Anyway, it works right when you patch in REAL serial and pcbsn (even if from a different kindle).