Still more of a pain than it should be - and also voids warranty. I wonder if they'll lock it down so you can't turn off the advertising on the cheaper model (the screen savers are advertising aren't they?)
Sounds like this would need pre-moderation unless you want to risk serious upset at some point. And pre-moderation is slow, plus requires knowledge of the subject (each book) not a task I'd want to take on!
Without knowledge of the book how would you know if the image was subtly offensive or a spoiler?
i have my kindle 'jailbroken' (which literally takes minutes from going to get the hacks, 1 to jailbreak and 1 for custom screensavers and implementing it) solely because I was really irk'ed by the standard screensaver images. I then used http://kindlewallpapers.tumblr.com to grab a bunch that fit my needs and loaded em up. Creating custom ones is simple as well.
My next step is to modify all of the .png screensaver images to also include my email address in case I forget my kindle somewhere.