I'm all for this. I was surprised when I initially saw that the /away command doesn't have a message when you /tell someone. A visual sign to show people standing near you that you're AFK is neat and all, but not very purposeful.

In addition to an automated response when someone /tells you, there could also be a feature that will automatically set you to /away after a certain period of time where there's no input. Say, after thirty minutes of the game receiving no input (via keyboard, mouse or gamepad), it could set you to /away. Just an idea.