Most people consider crushed eggshell to be equivalent or substitutable for oystershell (unless for one reason or another they refuse to feed one, or the other). (Some people have heavy metal concerns with oystershell; others have the same bioconcentration type concerns with egg shells, and/or fear encouraging egg-eating. The majority opinion on byc anyhow is that either one is fine, for whatever tha's worth)
If using crushed eggshells in place of oystershell, the main thing would be ensuring you have enough to make it always available free-choice.
I leave it out free-choice in winter too. Hens *do* have a regulatory appetite for calcium, so if they think they need it, I'd trust 'em. If they don't, they prolly won't eat much if at all.
Pat