I recall that those egg-eating snakes regurgitate the eggshells after their contents have been consumed. I suppose that means that either they cannot digest it or at least would be unable to digest it without it being ground into a more diffuse form like powder. Since those snakes are specialized in egg-eating, makes me wonder what that says about a more generalist species like us?
If you were to crush it into a fine powder, I don't imagine it would be harmful if consumed. If it were indigestible, it should pass harmlessly through the gut.
As far as getting calcium from food goes, I recall that more than once I have eaten the chicken bone inside of fried chicken legs. It wasn't easy and tasted somewhat like liver. However, I didn't get sick from it although I doubt it was good for my teeth.