I don't know if it's safe, but I wouldn't. I just chucked them out when mine hatched.
Raw eggs have the potential or already do contain salmonella. I'm not sure if incubated eggs are any different because of the fact that they were kept in high humidity and temperature, but I doubt it'd kill the bacteria on the eggshell anyway.
When my chicks hatched, some of them pecked at the eggshell (a really tiny bit which fell from when they pipped) out of curiousity, but yet, they even ate their own poop.
But if the egg was cooked (like boiled) and you saved the shell, than that's alright to feed chicks or adults.