Bill Myers wrote to tell me about his Video Bandwidth Protector for Amazon S3. As Bill says:
Basically, this system encapsulates a video file so that it can only be played when hosted within a web page served up by a particular domain. The script uses "security via obscurity" to protect the underlying URL from prying eyes.
Bill has put together a nice tutorial video for the protector.
The system is now available free of charge, but you are certainly welcome to leave a donation to support this project.