Oh, aside from my interests in the incredible edible box,
I cannot say with any kind of authority, but looking it up online it appears that Oyster shells have a lot of Calcium Carbonate in them. Calcium carbonate is the main component of the calcium supplements that I am familiar with- including many for humans. Calcium carbonate is chalk.
On these grounds, assuming that it is food grade, I would suspect it would be of benefit w.r.t. calcium supplementation but I would keep in mind there is probably a lot more in there than just the calcium carbonate. I would be worried about that.
Perhaps someone else has some personal experience?