Drywall – Safely hang a mirror that does not have hooks

At Home Depot (or Lowes) you should be able to find these Spring Loaded Mirror Mounting Clips made specially for hanging a mirror. They are no where near as big as a j channel and the top two have a small spring inside of them so they kind of grasp the mirror (and snugly fit on the glass).

You put the two without the springs in the bottom and the mirror just sits in them. Two more (with the springs) added to the top prevent the mirror from falling forward. Of course it would be best if you can get them into a beam in your wall. And you will barely notice the clips on the mirror once it is in place.

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.