The location of the Kindle Paperwhite's serial port has been confirmed:
(And prefixed and indexed)
This should open the door to unlocking the Kpw machines that have firmware 5.3.0 installed without a jailbreak or pre-installed.
Multiple ways should be possible, from manual command line access to setting u-boot to either network boot or to run an autoscript.
Just stick your solutions in this thread, I'll keep the prefix index updated.