Voted up, this is a great idea.
Several games in the past have had this option, where you had to wait out a countdown of inactivity to be able to log out. I think it would be even better to have some sort of combat timer as well, where if you were recently in any type of melee combat with a zombie, or if any weapon discharged a round in let's say a vicinity of 400 meters, you would have to wait for a minute or two instead of 30 seconds.