Yay, it works
I've managed to edit "extract_cover_thumbs.py" (from "ExtractCoverThumbs" app) code to process only files with .kfx extensions and dump images with correct filename (cover_ASIN.png) to needed location (
/mnt/us/linkss/cover_cache) folder and to save them as PNG and resize even smaller cover images to 1072 x 1448. Luckily ScreenSaver hack doesn't redo cover extraction if it finds already cached file for that book.
And I've created KUAL extension to run that script
