Thanks Emeraldo, I will see how I go in the next 2 or 3 weeks. If by then I can't tell the girls from the boys, I may have to reconsider this approach next time.
But I thought to do this it would need complete darkness. I have never grown with lights, but I have read many times where indoor growers have found that their plants have hermied when there is some light leak in the tent. I had assumed when reducing light hours to force an early gender reveal it would similarly require complete darkness, but maybe not? I can imagine how one could dramatically reduce the light with a tarp as you describe, but I question if that is enough, or does it not have to be full/complete darkness? I'd always assumed it needed full darkness so for that I knew that would be too tricky for me on the balcony especially with wind being present, a partial light reduction would be easier, somewhat. But we'll see what the next 2 or 3 weeks bring!