Hmm I would try pouring sugar into a shape that is flat, then painting one side with silver luster dust? Have no clue if it would work, but that's what i would try. Not sure if you could see through the sugar and if the silver would reflect anything, but worth a shot.
I just saw an edible "mirror" on one of the cake shows the other night....They made to form out of RKT and covered with fondant. It was basically an oval with a ridge around the edge. They painted the inside of the ridge with silver luster dust and then poured isomalt in. It looked good to me.=)