For that to work the light always has to be brighter on the gorilla side of the glass, otherwise it will be able to see through it. Since cell phones emit light, they might be visible even if the lighting is right. Good thinking, if they have a suitable window.
I’m aware. The brightly lit, covered (interior, iirc) exhibit in my hometown was hallway viewing, in which case it would have worked well. My perspective is admittedly biased.
Regardless, they have films which almost blackout viewing from one direction even with equal lighting on both sides. A solution exists...if they wanted one.