Cruel. Also awkward, impractical.
It would take SWMBO at least a week to eat all those pieces of chocolate, and the ring could show up at any random time. She might not get the gift until several days after Christmas and I might not be there. And she would spend those several days annoyed at me for not getting her a nice Christmas present. So rather than allowing the random draw, I'd have to pull out the right box and say, "Here, try this one."
One year I gave Mrs. Fast a string of pearls. We decorate our Christmas tree with strings of white pearl-like plastic beads, so several days before Christmas I hung her necklace on the tree, looking like it was just part of the decorations. On Christmas morning after she opened her other packages, I said, "You missed one." When she gave me a quizzical look I pulled the pearls off the tree and handed them to her. It was nice surprise and got a laugh.
So maybe you could try hanging the ring like an ornament, if it would blend in and go unnoticed?