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From twobob's transcription of seaniko7's video, you have to interrupt the boot by pressing [enter] at the right time (too soon puts you into u-boot) to get into the recovery menu.

Next step is to choose option 3 LOAD MMC0 over USB storage. Then you will need the Fiona password.
Option 3 makes MMC0 available for access via USB.

Connect Kindle to computer via USB, run dmesg to get mount point info for the MMC0 device. Let's say it's /dev/sdb1.

Unmount /dev/sdb1

This may be optional: mkfs.ext3 /dev/sdb1

dd if=/mnt/location_of/rootfs.img of=/dev/mmcblk0p1


All of the above that is correct is credited to twobob. All that is incorrect is my contribution and theory.


Edit:
Seaniko7 did this on a K3, not a DX. I do not know that the procedure has been done successfully on a DX. My own experience is that I can get into the recovery menu as he did (some choices are slightly different), but I am not given a chance to enter the password. Immediately after I press the 3 key, I am told that the password is incorrect and bounced out of recovery. I am not given a chance to enter the password. Note that once you are in the recovery menu there is a ten second timer to select an option. This can be seen on the video.

This approach seems to be one of two that have potential. Transfer through the internal serial port seems to be getting caught by an inactivity timeout. The other method - use of the recovery menu to mount MMC0 - is (for me) getting stuck at the point described above.

Additional note: In the video with a K3, there is a "recovery menu". On my DXG, the equivalent is labeled "developer menu". More interestingly, when in the developer menu, the DX screen shows the odd looking developer menu that megacoupe posted in a different thread. I believe he got the menu by rebooting and pressing keyboard keys. This method (interrupting the boot at the indicated point) is an alternate way.