Hmmm interesting idea!
I think your biggest limitations are going to be contrast and exposure bleed. First of all, typically mixing dyes of different colors (especially plant pigments which are not likely to be super dense), you get brown rather than black. For contrast, I think you'd be limited again by the lack of color density in your pigments, but as long as you're OK with pastel colors, that could work.
For exposure, at least based on the Wikipedia page you linked, you're talking one layer taking a 2h exposure. Trying to do say three layers would then take six hours, so whatever you're using as a mask has to be super absorbent or reflective to UV (think very thick black paper or tinfoil). Anything else, and your underlying layers are going to bleach somewhat.