While poking with my "bricked" Kindle Touch, I've found a way to enable "USB downloader" boot mode of Freescale i.MX508 MCU used in a device.
When mode is enabled, Kindle is recognized as USB HID device with VID 0x15A2 and PID 0x0052. It should be possible to connect to it via custom Serial Download Protocol designed by Freescale and described in MCU Reference Manual. Protocol allows to read/write memory, upload program image into RAM and start executing at given address.
(More info has been dumped to wiki page about Kindle Touch hacking
To enable this mode one should perform "hard reset" with pressed Home button. Hold Home button, then perform "hard reset" by holding Power button for 20 seconds and releasing it. Then wait about a second or two and release Home button. That's all.
It's easier to follow with Kindle connected to computer. After 20 seconds of holding Power button, Kindle USB disk should be detached from computer. It's time to release Power button. Then Kindle should be recognized as USB HID device. Now Home button could be released.
With loading arbitary code to RAM and executing it, this mode could be used as base for the ultimate unbricking solution, so I'm pretty excited about it. But it could be just a useless discovery if any restrictions will be revealed in further exploration.
Anyway, it is of no value for ordinary user right now.