Starting the screen saver from AppleScript is simple enough:
tell application "System Events" to start current screen saver
Even starting another screen saver than the default from System Preferences is simple, if you want one of the standard screen saver:
tell application "System Events" to tell screen saver "Arabesque" to start
But it becomes much more complicated if you’d like to start the iPhoto screen saver, and use another as the System Preferences default.
Here’s how I did it: