OK. Science nerd warning:
Just how "readily available" is Ca from oyster shell? It's not the most soluble material known to mankind.
Once you put it in the pot it is 100% readily available! At that point it is up to the plant to use it or not.
You are right Ray the calcium won't instantly dissolve even from the flour size particles and the flour particles will actually was through the media and out the drainage holes.
I am thinking that many plants like paphs and phrags get a great deal of benefit from having their roots actually in contact with calcium containing materials like limestone or oyster shell. I know P.kovachii does. I think the roots actually take calcium directly from the media more efficiently than absorbing dissolved calcium in the water. For that reason now I mix the oyster shell into the media. When the oyster shell has all dissolved away it is time to repot.