I pesonally never heard about a hair line crack in a chain plate
But i'm sure it's possible. I did heard about it on turn buckles and other part of the rigging.
But it's really no big deal to change a chain plate, it's basically just a plate of stainless steel... unscrew the bolts and replace it, you could probably have one made up at a local machinist shop or something but in sure you could find a source on the internet.
(if the mast is still stepped, make sure too loosen the stays/shroud)