Nope -- apps have to be self-contained .app bundles, and it's not permitted to write anything into shared operating system folders (which would be necessary to install a screensaver).

Apple is strictly enforcing that rule, too. My app was at first rejected for violating it. The specific reason given was: The application is creating files or writing to </Library/Preferences/com.apple.mediaio.DeviceSettings.plist> and <~/Library/Images>

It turns out that my app had nothing to do with this -- the responsible party was the IKPictureTaker class in ImageKit, which I was using to provide the "take a photo" functionality.

To Apple's credit, they made it very easy to email a real person about the rejection. When I explained that ImageKit is responsible, the rejection was rescinded within a day.