Just for the record, this works on Kindle Paperwhite 3 (2015) with 5.6.5. Only different is it had some password set for root, instead of '!', so instead of editing /etc/passwd directly, I've just chrooted into the main system and used passwd:
Code:
mkdir /tmp/main
mount /dev/mmcblk0p1 /tmp/main
cd /tmp/main
mount -o bind /dev dev
mount -o bind /sys sys
mount -o bind /proc proc
chroot /tmp/main /bin/sh
passwd # this will ask you for new root password, enter one
sync && reboot
Serial port pinout is exactly the same and it is easily accessible.
I have root and can use KUAL, but usual binary update packages from this site don't seem to work - kindle just deletes them from the usb storage for some reason. But installing them manually from source still works, so whatever.
EDIT: response to the post below: honestly, I would likely miss it anyway ^^ thanks.