Write your own script called say, down.sh, that controls this:
#!/bin/bash
SHUTDOWNDELAY=1
DELAY=$[$1-$SHUTDOWNDELAY]
echo Server is going down in $N minutes please save your work and log off! | smbclient -M winboxes
sleep "$DELAY"m
echo Warning: only 1 minute left before server reboots! | smbclient -M winboxes
shutdown -k +$SHUTDOWNDELAY
exit 0
Put this into a script in say /usr/local (which is on the path) and use this to manage the shutdown (don't forget to make it executable)
Call it with:
down.sh 5