I emailed Amazon customer support about this, and after a couple of days, they got around to misunderstanding my question and sent me the following unhelpful response:
"Thank you for writing Amazon.com Kindle support.
"Adding your own customer screen savers would be considered an alteration and would void your warranty. The Kindle limited warranty applies only to hardware components of the Device that are not subject to accident, misuse, neglect, fire or other external causes, unauthorized use, alterations or repair, or commercial use.
"I hope you found this information helpful. Our goal is to help you get the most out of your Kindle experience. If you need further assistance, please contact customer support at 1-866-321-8851.
"Thank you for choosing Kindle. Enjoy!"