You might be able to find a metal radiator mounting bracket or right- angled metal something, with a thin profile. Screw one side to the wall, and the other side can support the mirror, with the screws etc hidden behind the mirror. HiYou could paint it to camouflage, copying the underside of the mirror, so the one visible edge would be practically invisible. A single long continuous bracket would draw the attention less than two separate ones. Best to check the weight of the mirror, so you can check the bracket will stand the weight. Sydenham DIY are great for this kind of lateral thinking challenge. Go and rummage around. Good luck!