the answer is here...
http://prowiki.isc-csg.upenn.edu:16080/index.php/Using_a_ScreenSaver_over_the_LoginWindow
As for the local admin mentionned in there, well it's only to install the script. If you still think that you cannot install this, well I have news for you, go back to your basics!
You either push images to your machines (do this before pushing the image), or you netboot them (again do this on the image file), or you simply install each system one-by-one (then you actually have a local admin account on the machine).
AAM: Screensaver for Login Window