I have been pondering the exact same idea.
i get a type of rice from my local arabic food store that looks black before you cook it, but when you dilut the cooking liquid from it, it is a vibrant dark purple.I have been wonderign how this would work in a beer, and how much of this adjunct you would need to get the desirable color in a beer. it seems to have a very potent die and i thing about 15 percent of the thotal grist in a cerial mash would give the beer a purple color.
I encourage you to experiment. I am very interested to hear your results.
I am surprised to hear that someone else has thought of this, now i will never be able to say I did it first!
keep us posted, or maybe I will beat you to it!