My tank current tank has been running for almost 3 years with Rubble Rock and Coral Skeleton in the sump part of my tank. If a coral bleaches out, I just throw the calcium skeleton in to my sump! It works great as filtration. Where would natural bacteria rather live? An all natural coral skeleton (Where they have lived in the ocean for hundreds of thousands of years) or a plastic bioball? You be the judge.
I don't know about the overflow being filled though, as stuff builds up in there it may one day get clogged and you could end up with a hell of a mess on your hands!
Here is a picture of what the top of my coral rubble tray looks like...