Karl Kuehn posted yet another script to get the screen saver working over the loginwindow. I published a similar script in the past. I haven’t tested his version, but at first glance, it looks like Karl’s solution has a couple of improvements over mine:
1) It looks like it should work even with Fast User Switching turned on, and the machine is at the loginwindow with users still logged in.
2) Karl has implemented an elegant method to specify which screensaver module it will run; mine runs the default defined at /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/Engines Default.plist