Convey some message to all users

Nice question. Will give just an half answer... just a seed to see if someone can prepare something starting on this. I have tested this in my system, two users (ubuntu and romano, the two administrators).

I go to a VC and log in as ubuntu --- then I run

sudo sudo -u romano DISPLAY=:0.0 zenity --warning

(the first sudo switch to superuser, the second sudo execute zenity as the user romano)

from a virtual console, the message will appear in "romano" graphical environment.

I suppose that with a bit of mangling the output of w to get the DISPLAY and the users and looping over them it would be possible to have a working solution. Non-graphical connected users can be informed using wall.

I see really difficult to do that without superuser privileges, tough.