one way to invoke the screensaver already now would be to

1) enter the Behaviour Construction Kit 
2) create a new action and check the "global" checkbox 
3) add an AppleScript actor with this contents

tell application "ScreenSaverEngine" to activate

4) since actions can not yet be placed in the menu by the user, map a 
button in a behaviour of your choice to that newly created action.

Hope that helps.

Best regards, 
Felix