It is a one line command:
defaults write /Library/Preferences/com.apple.loginwindow SHOWFULLNAME -bool true
You can either drop that into a script and call that with a policy, or simply put that in the "Run Command" box on the Advanced tab of a policy. Scope the policy to the machines you want, set it to Every15, and away you go.
Its also possible to set the above as a System Level Enforced Managed Preference setting (aka MCX) so user's wouldn't be able to change it back (assuming the user was an admin) It grays the setting out in System Preferences.
its called "Display Mode" and is in the JSS MCX templates.
you guys are rad, thank you!