Here is a nice notepad addon. If others are also using it it can capture a send note and store it .. Cirk's Notebook
Also can across PopulusOfScientia at Curse ... Looks pretty interesting. Has a Message board that people can view and create messages on ...
But I looked around and could not find any mods that allow any changes to the MOTD. But there may be some I didn't find. I don't use any guild mods ... my guild is just a small handful of very close friends (and their alts).