Maybe I'm not looking hard enough, but how do you make a server message of the day?
Type whatever you want your MOTD to be in Notepad. You can type anything as long is each line takes less than 30 characters, because more than that will go off the screen. Save this as "MOTD.txt", and put it in the same directory as your Halo CE. Then, in your init.txt, anywhere before the "sv_mapcycle_begin" command, add the line:
Save the init.txt and restart the server.
Of course, this only works for dedicated servers, you can't do it in the regular server run straight from the game.