Bricked Gen3 reloaded from scratch(?)
Well, I did it!
I have a Gen3 that crashed during a firmware update and then locked-up at the first screen, LED on forever...
So, I connected the serial port and started hacking. I found the Samusung ARM vivi bootloater utility which has some diagnostic functions. I extracted the firmware from the update_kernel files supplied by Bookeen. Between the two of these I was able to determine that my linux kernel section was corrupted; I could even find the affected memory locations.
So, how to get it reloaded? First, I installed an SD card with the 1.2.789fmt update_kernel ready to go, should something good happen. Then I logged some of the vivi serial port traffic until I figured out what was going on. I managed to load the kernal into RAM, then boot from RAM and it came up! I quickly pushed the enter button and it started the firmware update process and... it worked.
So, without further documentation etc. I'm not certain if this is a universal fix, but if you have a thoroughly bricked Gen3 Cybook then it's worth a try. It does take some skills (I am an electronics engineer) and tools (3.3 Volt to RS-232 converter.
It appears from the forums that this is the first time this has been done, so there may be some hope after all. LEt me know if you want more details.