Hasn't Draco been with Lucius for the past hour? It would be one thing to steal the wand of a generic death eater, another entirely to steal a wand from under the nose of the head of the majority of the Wizengamot. Lucius seems to be well versed in the art of plotting and counter plotting, and getting Draco's wand from him and back to him without Lord Malfoy knowing with, at a maximum, five hours of planning, would be an extremely challenging feat even for Dark Harry. Still, I don't think that it is impossible.