It may not be possible to display the message on the login screen directly. But you can send a message prior to logging in. Add to your
greeter-setup-script=xmessage -button ok -default ok -center "Hello world"
Be cautious as wrong option in this file can break your boot procedure.
Mouse will be unavailable on this stage, but you will be able to close the message with Enter.
xmessage can read the message text from the file or can have custom timeout. Check its manpage for additional options.
You may check https://wiki.ubuntu.com/LightDM for additional information about
lightdm.conf and greeter customization.