I might be mistaken, but you might be able to use ProPresenter with stage display view. I know that you can trigger presentations/videos via midi but I'm not sure if it will sync time. That being said, you can create an event the will start a clock or countdown. Just a thought.
Figure 53 (who make QLab), make a small utility called TimecodeDisplay that does exactly what you want. All you'd need is a display somewhere that the actors can see.