there should be a clear 5 second countdown from the official timekeeper over the PA
A good idea, although making sure it can be distinguished from the other countdowns may be important. Just as an example, I recall one fight (think it was a three-way at last year's FW champs) where everyone could hear a 10 second countdown through the PA and the robots stopped fighting after it thinking that was the battle-ending countdown, but it was actually the immobilisation countdown for one of the robots.
As long as the roboteers know who/what the countdown is for then it should be fine. Same goes for the audience; with two countdowns in place already we just don't want them getting confused as to which one is which.