Peder

I think this is right up there with making a motorcycle out of wood. Yes, you can do it but why bother. Anyone working with hard plaster should have a supply of plaster washers (they are cheap). As for cutting all the divots in the old plaster and on the fracture edges no less, that is a relatively weak support. Better to move back from the edge, drill a small pilot hole with a 1/8 masonry bit and run in a screw with a plaster washer. The washer spreads the lifting force over ~2" of surface without compromising the strength of the surface. It is true that the washer stands slightly tall of the surface but that should be skimmed over after taping.