We do not support the Kobo here. You should try in a Kobo forum.
But for the kindles, it would be possible to do what you suggest with a background script (or native C program). Instead of running in the background, the "book cover to screensaver" script could be launched with hotkeys using the launchpad hack. It would take some research, development, and testing though (from somebody who wants to invest their personal time in such an endeavor).
Your options are to convince somebody to do it for you for free, or to pay somebody to do it for you, or learn how to do it yourself (and be sure to publish it here when you are done).
