When Ubuntu was using gnome I was able to create a popup banner like the banner below before the login screen using zenity in the /etc/gdm/Init/Default. The line of code would be like this:

if [ -f "/usr/bin/zenity" ]; then /usr/bin/zenity --info --text="`cat /etc/issue`" --no-wrap; else xmessage -file /etc/issue -button ok -geometry 540X480; fi

How can I accomplish this with Unity?

NOTICE TO USERS

This is a Federal computer system (and/or it is directly connected to a BNL local network system) and is the property of the United States Government. It is for authorized use only. Users (authorized or unauthorized) have no explicit or implicit expectation of privacy.

Any or all uses of this system and all files on this system may be intercepted, monitored, recorded, copied, audited, inspected, and disclosed to authorized site, Department of Energy, and law enforcement personnel, as well as authorized officials of other agencies, both domestic and foreign. By using this system, the user consents to such interception, monitoring, recording, copying, auditing, inspection, and disclosure at the discretion of authorized site or Department of Energy personnel.

Unauthorized or improper use of this system may result in administrative disciplinary action and civil and criminal penalties. By continuing to use this system you indicate your awareness of and consent to these terms and conditions of use.

LOG OFF IMMEDIATELY if you do not agree to the conditions stated in this warning.