What a waste of milk and of money, and what a stinky, sticky mess!
If the goal was to make water reflect more light, why would they not just add something harmless and cheap to water?
I think you are wrong...but I don't know for sure. It would not be the first time Hollywood did something idiotic.
As to the print, if you were going to do this, how would you do it?
You certainly would not scratch your negative if you wanted streaks of light rain!
If I had any sense (which I may or may not), I would not alter the picture on the negative at all. I would make an underlay of some sort for the neg. Black lines on a piece of glass or film would do it.
wildbill is correct. I'm in the Fx business. That's exactly how you do it. And it takes less wind that you would expect to get the rain to go sideways. A large fan from Home Depot and a black background would do the trick. Then all you need to key it, is Photoshop.
My first thought when I saw the photo was strings stretched across the set. Could also explain the "wind driven" effect. Both upturned corners of the kimono are quite close to a rain streak. Like I said, just a thought.
Whether or not anyone here would scratch their negatives you're discussing a photo that's 100 years old nad it wasn't uncommon for the photographer to do that.