MythMessage is a replacement for mythtvosd, which provides the ability to send a message to a remote frontend.
I could't find any information about MythMessage so I thought I would start a page for what I have learned so far. This could all be completely wrong!
As far as I can tell, in MythTV 0.25 a message command has been added to the Frontend Network Interface. Therefor it is very simple to send a message. Just telnet to the IP of your frontend machine (on port 6546).
telnet <frontend> 6546
You can even do this in a script simply by piping the message to the telnet command.
echo "message Hello World" | telnet <frontend> 6546
When a message is sent to the frontend, it is displayed centrally on the screen with an OK button. The message does not clear until OK is selected.
According to Little_Gems#mythtvosd the old mythtvosd method allowed differnt types of messages to be sent.