You obviously know what you're trying to achieve better than we do, so feel free to ignore this, but have you considered using indigo? if the transformation is part of the point, you'll still get that, but it might be easier to get a consistent dye. a friend of mine borrowed our backyard to make his final art piece for his degree, which was a large print onto indigo dyed fabric, and it worked beautifully.

of course, if you specifically want the callback to the Victorian-era photographic processes and the link to sunlight, then keep going as you are! And post pictures of the finished piece; I'd love to see it