Ok, I decided to stop looking for unusable software, and instead found conversion table between Polish characters and HTML entities - and replaced them, one by one, by hand, in text editor.
And Cybook does indeed support Polish letters in HTML, when served to it in this manner. I stand happily corrected
Here's the table, if it's useful to anyone:
http://mynthon.net/howto/-/html%20-%...20entities.txt
Sorry for interrupting... so where's this new firmware again? :P