Just a suggestion, have you thought of using something like an oval ceiling medalion around your current light fixture, but adhering it to the wall to cover up the hole? Maybe something oval shaped that would sort of match the mirror?
I'm not sure how that would work since I can't actually see how that light fixture attaches to the wall. Do you have a picture? Perhaps you could even take a piece of wood and router the edges a bit and paint it white and nail & glue it to the wall?
I like your wallpaper-- and I usually don't like wallpaper. Since you have that sort of curved leafy pattern, something like this with the middle drilled in the shape of the light fixture surface?
http://1.bp.blogspot.com/_CVAlg9ckpVY/S31MDrlnKII/AAAAAAAAEH0/XTSdzGAX2W8/s400/whtorigorn.JPG (sorry I can't get it to insert images right now-- browser is not cooperating).
Or something like this (although the red on this is not good and I don't know if the gold would clash with the polished chrome look)
http://1.bp.blogspot.com/_CVAlg9ckpVY/S31MCGjZ-nI/AAAAAAAAEHs/_suT48iyn6k/s400/finished+medalion.JPG
You may want to go with something that is just simple but that matches the shapes in the room.
At first I was just thinking of a face plate or wall plate that they put on the wall when a wall switch is removed, but those can look kind of ugly. If you don't want to ruin the symmetry and look, the ceiling medallion or some sort of escutcheon might work. Run it buy your fiance and see what she likes.
I believe searching Lowes for blank "wall plate" (quotes only around wall plate) turns up some options if you just want to cover the hole.
I forgot to ask, did you do anything to the edges of the holes you cut in the shelf to seal them so they weren't so rough and also to prevent moisture from getting in if there should be any leaks? I know it probably isn't a big deal or anything, but I would want to seal it somehow and also get rid of rough edges so it doesn't catch on anything. But that's just me, and I am a newb with stuff like this.