When I saw CATS last month in Japan in CATS Theatre, since there's no orchestra pit, they put a tiny little light on the balcony, and a green light flashed for every first beat, and the red light flashed for all the other beats, and they only had it working only when they NEED the conductor's cues. (like fermata, rests, and long notes that are held) Once a song got going and established a rhythm, the lights stopped blinking.
I thought it was pretty clever of them.
"Hey, you! You're the worst thing to happen to musical theatre since Andrew Lloyd Webber!"
-Family Guy