If its possible maybe set up a icon where it tell you how long the person is afk for.The timer icon could be found when you inspect a character and it shows how long the person didn't move for.
AFK timer going up?
The timer goes up when the person stands still anf doesnt move. And resets to 0 when moved.
Negatives?
The timer probably uses to much data to run and may crash result in the timer not to go up or reset
Useful?
Probably not to useful