Would copper plated lead not suffice?
If the goal is to not ingest lead, the copper plating being much harder than lead eliminates the deformation of the pellets and exposed lead travelling through the barrel and choke and into the turkey. Also, unless you make an incredibly poor shot, I don't see how you get more than maybe 1 or 2 stray fully plated pellets in the very upper portion of the breast.
If the goal is to not put any lead onto the landscape I understand, but would also counter with the fact that any pellet that would end up landing and remaining in the field will incredibly scattered and are coated by copper that will take 100+ years to corrode to the point where lead would be exposed, and will more than likely naturally be covered by leaves, grass, dust, dirt, etc. which decreases the exposure to factors that will cause deterioration meaning it will probably be incorporated into the soil long after this world is destroyed by nuclear weapons... Just a thought.