I would think it takes several hundred years before coral skeletons are incorporated into live rock like we're used to seeing, maybe longer.
White coral skeletons like I used to keep in my FO tank years back make great green algae magnets, though.
Tough to keep clean.
I just don't know that they would be of any benefit to your reef tank in the same sense that "real" LR is. Even the old dead corals that make up part of our LR is barely recognizable as such any more.