Out of curiosity, how long would dried, crushed eggshells keep? Is this something that could be prepared while the chicks are growing up prior to laying?
Eggshells don't have to be baked or microwaved, but many folks find they crumble easier when they are. I'm not sure how long dried eggshells would last, but unless they had a lot of eggy residue there shouldn't be much to spoil. Perhaps if kept in an airtight container, maybe in the refrigerator, they should last a long time without spoiling.
I give my eggshells a quick crush every morning and freeze them in a gallon ziploc. My chickens don't need them yet but I will have plenty when they do.
Do not - REPEAT: DO NOT - keep undried egg shells in a covered container. I tried to get all fancy by keeping them in a covered container under the sink and they molded.