In the new patch (171.0) we can now finally set a MotD. According to this post by the devs, this is how it’s going to work: Put the following text into your GameUserSettings.ini and change it to suit your needs.
[MessageOfTheDay] Message=Welcome to your new home, on my ARK! :D :) <RichColor Color=1,0.1,0.1>Red Text</>! Duration=20