It looks like Cool-er can be updated with Росketbook 301 firmware (which is considered to be the best among the Netronix' clones). Those interested do the following:
1. Google and double-google to make sure that Росketbook 301 rules.
2. Google for < "FR-TO-PB.EXE" multiupload >.
3. Get the FR-TO-PB.EXE self-extracting archive, extract its content to an SD card to be used for the update.
4. Find the fr2pb.pl file in the myfiles folder of the update set, open it in a text editor, locate the following lines
!grep(/mtd5: 02000000 00020000 "app"/,@mtdtable) ||
!grep(/mtd6: 1d440000 00020000 "other"/,@mtdtable))
and replace them with
!grep(/mtd5: 02000000 00020000 "app"/,@mtdtable))
5. Save the file and update Cool-er using the SD card.
6. Enjoy.

Note:
- they say that Cool-er will not be updateable anymore after this update.
- do it at your own risk, only if you are ready to get your Cool-er bricked.
- if you started the update but after 10 minutes you are not sure that it is in progress, you have failed. If your previous firmware will not install after 2 attempts, install booq's (also do it twice, if the first attempt fails), then yours or try again with this one.
- there are at least two people who have made it (judging by the-ebook.org posts).
- the default interface language will be Russian, but it can be changed to English, German or Hebrew.

PS More things to note:
- I was informed that the internal memory of Cool-ers with 1 Gb onboard decreases to 512 Mb. Personally, I think the gain is worth the loss;
- One can install the latest version of Росketbook SW (v15.2) after this update (which is 14.2). However, v14.2 is considered to be more stable.
That does look good - I'd noticed that the booq firmware might be compatible but had yet to try it out. I may see if I can brick my Cool-ER again tomorrow
Thanks for the tips!
Added "More things to note"
Very interesting.

Something I noticed, while glancing through the files:

There seems to be an image for the EB600EM, but not the EB600E. So that may be the reason why for some it bricks the device.

This installation does not have the usual(?) TGZ files containing a whole system - does it copy itself onto the device?

If so, there is no backdoor anymore, it seems - it has been removed from the rcS file.
Could be reinserted, I suppose.
Installed this onto my Cool-ER... It is way better than I'd imagined

There's no real need for a backdoor - you can get an actual terminal running on it, and practically everything is configurable via the settings or config files...
Damn.
I'm jealous. I'd love to try it too, but I don't dare trying it, because I know the hardware is different :-(

I suppose I could try to replace the U-Boot files, but I doubt this would be a good idea.

I guess I'll have to content myself with rummaging through the files...
The booq firmware linked here through these pages should still work quite nicely on an EB600E. Plus, I think things like dictionaries were included too.
codeMonkey - My congratulations, welcome to the club! I hope you'll find the firmware good enough (anyway, there is no known way back, as you remember).

Cyberman tM - I had to add that note about bricking possibility just because some people cannot follow instructions. The two cool-ers i saw turned into pосketbooks survived the operation all right, with no bricking.
However, I did not know that there were as EB600EM, so EB600E Cool-er models.
Nonetheless, it may be interesting for you to have a look at ShineBook firmware, too: http://shinebook.eu/scripts/Shinebook-vfat-eb600em-v2.2.7-20101123.rar
It has a German dictionary installed (booq has Spanish). The dictionaries seem to be not replaceable. I used to install that version with no problem at all. I made sure that it could be replaced with booq or cool-er firmware if you do not like it. Still, all the risks are yours.
Problem is, I don't have a CoolEr, but actually an Elonex. :-/
Almost the same - but not quite.

I'll take a look at the Shinebook firmware, thanks.

[edit]Don't you like shuffling through new firmware files? :-)
I see this firmware uses SKIN files. And it has the Jam VM. Very interesting.
I was willing to risk bricking it to see what the pocketbook firmware was like - but yeah, it's fantastic. My reader had been feeling a bit old and outdated recently, and this has given it a new lease on life.

Have you updated to any of the newer firmwares after this one?