After spending the weekend trying to figure it out on my own, I finally bugged Nintendo directly about it. Turns out that, according to the company, the little status message that appears above your Mii's head in the Friend List is it.
Even as status updates go, ala Facebook and Twitter, it's short and sort of pointless. There are no alerts when your friend's update their status the 16-character limit is beyond restricting.
Nintendo still isn't talking about future plans for more elaborate in-system messaging between 3DS friends. I sure hope it's soon.
I know Nintendo wants to protect us from online predators, like Apple wants to protect us from Flash-based sinning, but I'd like to think they could come up with a better approach.
At least bring back some form of Pictochat, Nintendo!