Password protect DVD

Here's an interesting one.

So we all know we can password protect our computer screensaver, so that if our screensaver is on we need the password to return to the computer work environment.

But can a DVD be authored to hold a password, so that while it's showing, you need a password to stop it???

I've had a client ask me to do this, and while I've said I'm 99.9% sure this can't be done, I promised I'd enquire.

Can anyone surprise me?

Cheers

Mike