I haven't tried yet, but shutdown -k "message" should work. However, the message would not flash on the GUI screen of all systems. Moreover, all users need to be connected to same host. Rather, you could create on indicator, with python backend, listening messages from a particular socket or port on the same network (by checking same IP range). You can also borrow the concept of presentation remotes as impress remote.
Please do share if you get some way. Thanks for A2A.
