It is an option while playing on dedicated server, but when playing peer-to-peer it is very uncomfortable, because you need to run the singleplayer once more and open for internet what is taking a lot of time.
When there would be built-in pause it would be a lot more efficient, bacause you wouldn't be wasting time for reconnection.
As I guess, it isn't so hard to implement, cause the game has a pause already implemented in singleplayer. The only thing that need to be done to achive that is adding button which would turn pause on and off and when the game would be paused it would automaticaly send information to other players that the game is paused or not.
With lots of respect
Vantyndur