I’m guessing at the moment that Apple Remote Desktop 2.2 works only with Tiger (and in any case costs 299USD minimum and isn’t open source). In any case, the administrator’s guide  states on page 82:
"Apple Remote Desktop can lock a computer screen. When you lock a computer screen, no one can see the desktop or use the mouse and keyboard on that computer. By default, ARD displays a picture of a padlock on locked screens, but you can display a custom picture.
You can continue to work with computers using Remote Desktop after you?ve locked their screens. However, the screen becomes unlocked if you use the Observe, Control, or Share commands. "
So an API evidently exists, although - as you say - it may not be “available” in the sense of “published”.