what I would do is snap a rectangle slightly larger than the damaged area,then screw a few plaster washers just outside your lines(to prevent damage to your good plaster when you cut along your lines).Score your lines w/utility knife and then cold chisel the damaged plaster out.Then,along the edges shim out strips of 1/4 ply wood and fasten to accomadate a drywall patch,then fasten your drywall to your plywood strips and tape it.Have fun!

If you want to patch a hole in plaster the best way to do it is to use drywall, or Blue Board, and plaster over that.

If you use drywall you will need to paint some PVA over it first so it will adhere.

Brian Ewing
Ornamental Plaster Resources