Somewhere near the beginning of each Starsiege Assault .cs file, add the line

$gameType = "Assault";

Then edit the motd.cs file in /Starsiege like so:

if($nameOfTheGame == "Assault")
{
  $motd = "We apologize for the inconvenience";
}
else
{
  $motd = "Forty-two";
}


This sets one message to play when Starsiege Assault maps are being run, and another message for all other maps.