The way to do it is just to write a script:
Code:
#!/bin/bash
ssh user1@host1 env DISPLAY=:0 xmessage "shutting down soon" &
ssh user2@host2 env DISPLAY=:0 xmessage "shutting down soon" &
ssh user3@host3 env DISPLAY=:0 xmessage "shutting down soon" &
ssh user4@host4 env DISPLAY=:0 xmessage "shutting down soon" &
This is a pretty awful way to network notification though.
Not sure about a better way.