Mounting a Fireplace Mantel - Steel Studs
A plywood backer is a great starting point. From there you could bore some hole edgewise in the live edge mantelpiece and install the anchors mounted into the plywood. If done properly and carefully your Live Edge mantel piece should be able to withstand much weight plus its own weight without moving. One thing to keep in mind is the height to distance ratio of your mantelpiece. Combustibles going over a fireplace have a ratio of how much horizontal distance off the fireplace to how vertically high above the fireplace your combustibles are.