Pretty self-explanatory. The normal MOTD gets sent to everyone in the guild, but an additional MOTD can be set for Council, Lieutenants, or normal Guildmembers that only they could see.

There is a guild suggestions sticky; you should post this idea here.

But before you do that, please give some examples of how this could be useful. I can imagine how this would help... maybe... but I wonder what prompted this suggestion.

Alternatively, you can use an online guild forum, although that's obviously less convenient as it resides outside of the game.