I think air-brush people use the frisket film.
They also make liquid Frisket. It's a lot like rubber cement. I've only used it a few times. I painted it on to an area I didn't want to bleach, before sepia toning. The area wasn't uniform ( an area of reeds) so exactness wasn't too important. If it's a more uniform area, you'll need extra care, so the toning is exactly where you want it. When you're finished, it rubs right off with your fingers.
I don't know the image you're thinking of but, have you considered selenium toning the whole thing, and hand-painting the deep brown? If you're using a FB matte paper, it may be easier.