I do know people feed eggshells to chickens. However, it seems to me if a vegetarian is eating an egg, there wouldn't be a problem with feeding the eggshell to the chicken. Just my view. I will ask my friend.
I have not kept any for that long. This year I want to, so I am interested in the answer. I know once they get to layer age, give them layer food (I use pellets, but some use mash. Not sure if there is any difference to the chickens) I think I read you need to switch them to a high protein...