Try sending this script using root in the "Send Command As" field:
tell application "System Events"
substituting the approprate login name and password for the dummy information between the quotes (i.e. don't include the braces). Depending on the version of Mac OS X on the workstations, you may have to remove one of the "keystroke return" lines or add another one. This presumes that your workstations are set to display the login window as name and password rather than as a list of users. I haven't found a script that works when the users are shown as a list.