Plugs work great on a rack. For attaching to existing rock, not so much, but with an encrusting coral, eventually the plug part gets "absorbed".
For rock with pre-drilled holes that match the plug stem, the would probably be awesome. Maybe we should standardize plug stem diameter (ile 3/8" or so).
How about threaded plugs that you can attach to prethreaded holes in an artificial rock?
Or... threaded plugs and a matching threaded plastic "nuts" shaped like rock rubble that you can pre-glue to a piece of real live rock. So you can get a dry piece of rock, pre-glue on a bunch of rubble-looking nuts, then put that in the tank. Then as you get frags, you mount them to the threaded plugs and then you can thread them into the sockets. (or not even threaded, just rock rubble with holes that match plug stems).
V