Nearly bricked my cooler but upgrade thru USB cable is definitely possible using the eSlick cmdfile tool.

I did extract the u-boot from the archive and sent it to the Cooler using cmdfile -u and the nand was updated. I guess i simply choose the wrong one because the Cooler started to behave odly ... So if you want to play with it better to start with the fs rather than the u-boot

Back to normal after a re-install of the firmware. Kudo to the SD upgrade for being so robust.

P.S. Do not try if you are not sure
Cool-ER firmware grants us so much flexibility. I was able to replace the es3.ttf file with my own UTF-8 font then flash from SD, and it works.
Thanks for sharing the info on that with us labzuk - I plan to have a good play with it as soon as I get a chance - I've already got all of the dev stuff I need from the eSlick site.

Note for anyone curious - the cmdline tool is, I think, linux only at the moment.