Message window at login window

I have a startup item that runs softwareupdate -i -a, deletes the startup item, and then reboots. I want to display a message at the login window asking the user not to login until the computer reboots. I have tried using the bash osascript command. However it does not want to allow it. osascript -e 'tell application "Finder"' -e 'activate' -e 'display dialog "My message"' -e 'end tell' But I get this message osascript[643] : 3891612: (connectAndCheck) Untrusted apps are not allowed to connect to or launch Window Server before login. _RegisterApplication(), FAILED TO establish the default connection to the WindowServer, _CGSDefaultConnection() is NULL. Bus error Does any one have an idea of how to display a message window at the login screen? I know how to change the login screen message. I want something the users will have to move or click on before they can log in.