We have used glass back splashes many times, and have found that it is critical to have the proper cement board underpayment and the recommended white thinset under the glass tile. I would strongly vote against trying to put glass tile over wall paper.

Are you thinking tile or solid piece of glass?

I agree with @FINNE. Take all the wall paper down. Do the job correctly and you will not have problems in the future. There is no cutting corners in construction and that will benefit you down the road.

Again, I see no reason you can't paper behind a solid piece of glass...make sure it's tempered and recommend starfire glass to prevent color distortions.

I was actually thinking of using wallpaper as the backdrop and covering it with the glass.

Would not work well with tile.

This would be possible with a solid piece of glass. You would need to adhere the wallpaper to the back of the glass first, and then use thin set to adhere the glass to the wall. You do run the risk of having the wallpaper/glass interface fail.

Would mirror clips work to attach the solid glass to the wall? A clear silicon could be added to the edges to seal it from water. I've never tried it, but this is an idea.