I like the "alarm" idea - like museums have if you get too close to an exhibit. It is like a virtual fence.
In all honesty, I personally feel it is time to make cruise ships "fall-proof"
(1) because falling is just too deadly.
(2) because these staterooms are inhabited by all kinds of different people, not just mature adults with no history of depression.
But on the other side of the argument:
(1) the Golden Gate is not fall-proofed
(2) New York City is not fall proofed
(3) the barriers that are in place ARE foolproof against accidents and that is legally enough
It is purely a moral argument, and one where you are erring purely on the side of caution. However, Las Vegas has chosen to limit the number of balconies and windows that open. They did that for a reason.
One thing you could do is make the open decks fall-proof, and then offer 10% of the staterooms to be fall-proof, and keep the rest of the staterooms as they are.