I have read that ducks love to pick spent birdshot off the bottom of their habitat, and the incessant rubbing of stones against the shot in their gullet gives these ducks lead poisoning.
Mollison's Design Manual doesn't mention this mineral milling action as one of the outputs of a chicken, but I wonder if it could be useful. Particularly, I wonder if mixing appropriately-sized charcoal pieces into a bird's grit would be worthwhile, and what effect it might have on the bird. I also imagine supplying grit of two different hardnesses, the softer one containing trace minerals needed by the bird and/or the soil, might be a less-laborious method of producing rock flour.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.
I think however the minerals are provided to the animals they will end up one way or the other back into the soil, especially if you bury the eventual carcass. I wonder if chickens would be choosy if you offered them a variety of grit?