Can you send a message across the network?
According to this metafilter post you can send a message using Applescript (pre-installed on every Mac) and Growl (old version is freely available). The relevant part is quoted hereunder.
tell application "GrowlHelperApp" of machine "eppc://xxx.xxx.xxx.xxx"
register as application "myapp" all notifications {"message"} default notifications {"message"}
notify with name "message" title "title" description "description" application name "myapp"
end tell
Here xxx.xxx.xxx.xxx should be substituted by the IP address of the sender.