I don't think it's a case of not being possible, just that nobody has bothered to write a script to automate it.
If you wanted a kludge you could save the cover image as your only screensaver, then it will display each time you switch off. But you would need to change the screensaver each time you start a new book. If you keep the same filename (eg cover.png) you won't need to restart the Kindle each time.
So all it would really take to automate it would be to extract the cover image when the book is opened, and save that as cover.png in the screensaver directory.