I'm looking for a drywall finish that will hold a magnet
If you really want to securely hold objects that heavy to the wall using magnets, I think you need to be working with sheet metal on top of the drywall (or in place of the drywall).

The magnetic force will fall off very quickly with distance away from the plate, so even skim-coating with plaster will dramatically reduce the holding power of the magnet.



If you want the surface to blend in with the surrounding walls (you didn't actually state the finish on the walls, but I'm assuming it's painted sheetrock or plaster), try simply painting the steel sheet to match. The texture of the finish might be slightly different, but I'd imagine it wouldn't take away from the feeling that it "blends in" with the surrounding walls.

As others have stated, I'd also consider a polycarbonate mirror rather than glass.