I did research how well mice can tolerate copper before I fed hog food to my mice, and I was able to find info from scientific studies that indicated mice can tolerate a lot of copper. Admittedly I'm not 100% certain how it affects the snakes, but I'm not the only person who has fed my rodent colony with hog food, and so far no one is reporting problems.
I assume that since it is safe for people to eat these hogs that have been fed on these high copper hog feeds, that the amount of copper that stays in the animal is not nearly as high as in the food they feed them. So the same is probably true for rodents.
Perhaps the issue with sheep has more to do with them being super sensitive to copper? I know copper is used in certain medicines for aquarium fish, but you can't use it if you have certain species of fish because it is toxic to those species, but just fine for most fish.