First off, to satisfy my masochistic side: I was stupid/reckless enough to try almost the same stunt he did.
Same fate, of course. Device bricked.
Fortunately, somehow I managed to get the Cool-Er firmware to install itself on the device, basically unbricking it.
I have yet to truly test it, but preliminary tests seem good.
Strangely, all I did was following the instructions of Cool-Er. Maybe the hardware of my EB600 is different from Tonyv's?
All I did was (repeatedly) pressing the center button as well as the third button from top(also labelled return) and pressing Reset.
Perhaps this wipes the internal memory and awaits bootable sources on the SD card?
If so, then an interruption (for whatever reason) might seemingly brick the unit.
Doing so again, this time while the eReader was connected to the PC (the Cool-Er instructions are ambigous here, they talk about disconnecting, then connecting again?).
At first it didn't seem as if it actually worked - when all of a sudden the new logo flared up.
The rest was as expected - on-screen information about update/install progress, afterwards it worked again.
Of course, I couldn't leave it at that. I tried to install the CyBook firmware, using the same method.
Apparently it didn't work. Device bricked again :-(
So did the same again, with the Cool-Er firmware. Seems to work the second time.
Either the Cool-Er firmware update is somehow specially built for this purpose, or it's just luck that this one works while others don't. It seems as if the Cool-Er actually compiles the entire firmware. (The firmware package content looks like that too, though I didn't really check too closely.)
Interestingly, when connected to the PC, it still identifies itself as Elonex EB600.
So, what it boils down to is: The Cool-Er firmware update might be able to unbrick an EB600 if it is lacking a firmware. It takes a long time till it responds, perhaps it IS coincidence that it works?
In any case, it worked twice on my device, and I'm happy. Hopefully no one else needs to read this.