He spent a couple of weeks chasing an answer but in the end they said that who set the message was private account information that they would not give out to someone else (great cop out isn't it :) ). And there we are stuck! A couple of guildmasters left for more hardcore raiding guilds, so maybe, hopefully it was one of them . . . but I guess we'll never know. Can you think of anything that could be done to avoid this or controls to avoid this kind of offensive message?
The only way to avoid this situation is to make sure the people who have access to the GMoTD are trustworthy and responsible people. If your own officers prove that they aren't, then the guild leader can actually modify the permissions so that only he or she can change the message. But there is no built-in censorship to this feature aside from the one that sits in everyone's interface panel, and even then I don't know if the profanity filter actually affects the GMoTD.
Since you (perhaps wisely) didn't specify what the message actually read, it's difficult to judge the severity of the situation. A lot of guys who are otherwise mature adults sometimes joke around in a way that could be construed as homophobic, but typically no offense is meant. It can be good-natured smack talk, but it can also go too far and get truly offensive. Since another officer went to the trouble of contacting Blizzard about the message, I'll have to assume it's the latter.
Even though this person wasn't "caught," I'd say the officers should meet with one another and ask the offending party to step forward. Based on your e-mail, they may have already done so. However, it's entirely possible that none of the officers know who did it. If an officer's friend or roommate snuck onto the officer's machine while he or she was AFK, then that person could have changed the message with no one the wiser.
More important than who did it is how the officers respond to the situation. An apology should be made to the guild's membership. It should also be communicated that effort was taken to look into the matter, even if the results were inconclusive. If members were upset by the message, they need to know that the officers weren't passive or dismissive of the insult.
In the end, it's not necessarily a huge deal and shouldn't be made into one by extended periods of finger-pointing. Sometimes overreacting to a situation can be just as bad as not reacting at all.