So I've had a chance to un-demo both K3 (Kindle Keyboard) and DXG earlier today and just wanted to share my experience.
The K3 was easy as the K3 Noob Debrick thru ATK worked wonders: https://wiki.mobileread.com/wiki/K3noobdebrickatk
It took a while for the K3 to reboot afterwards and I was afraid it didn't work, but once it rebooted (I say let it sit plugged in for an hour after the programming is done and if that doesn't work -- hold the power switch for a while) it booted into normal K3 software and after Factory reset and jailbreak I was good to go.
The DXG is a bit different in a number of ways. First of all, the DXG demo is just a bunch of rotating pages/images, while the K3 lets you sort of exit the demo mode to play around Kindle with some menus in Settings disabled. I also could not get the ATK to recognize the Demo DXG thru the Power/Home combination (as per wiki article before). I was able however to get into recovery, but I couldn't get the update_restore-os.bin to work -- it gave me errors during update.
I first ran the Sir Alex's KindleDiagnosticTool.exe and got the root password for my Demo DXG. I then copied the retail DXG kernel and rootfs.img to the Kindle thru USB/Mass Storage and unmounted the Kindle.
I then popped the plastic cover, got the USB/RS232 TTL adaptor plugged into my Mac and I could get into the login prompt once connected to Demo DXG.
In the serial console I logged in with the login root and the password from the KindleDiagnosticTool.exe and ran the following command:
dd if=/mnt/us/mmcblk0_7688.dmp of=/dev/mmcblk0 bs=512 count=7688
dd if=/mnt/us/rootfs.img of=/dev/mmcblk0p1
At the end it started throwing errors like "Buffer I/O error on device mmcblk0p1, logical block nnnnnnnnnnnnn" -- as I'm guessing when it could no longer load next page/image.
After reboot with the hardware power slide I've got the working Kindle DXG with the 2.5.8 firmware.
A few photos which might help now..
The plastic cover snaps location:
So you could see where to press on the cover to disengage snaps.
Wires on the serial board:
Green to RXD, Yellow to TXD and Red to GRD. WARNING!!!
On the photo the black wire connects to 5V output on serial board, as per knc1 suggestion it's better not to use the black wire at all! I have tried it and it works without black wire.
Wires connecting to Kindle DXG:
Black on P1 and red on P4. I've carefully stripped the plugs of the plastic and wrapped everything with tape so I could just touch the contacts on DXG instead of soldering.
On Mac I've done the following to connect to the serial port:
sudo cu -l /dev/tty.usbserial -s 115200
With Windows you should try to use putty and if you're on Linux you probably won't need my help on that.
Once you're connected you can either dd from the user mount partition (as briefly explained above) or expose the Kindle to the USB host and dd over the USB cable.