I suspect that it is purely a literary invention. The core idea of the story is a variant of the "Adventure of the Blue Carbuncle", where a stolen gem was eaten by a goose. For the new plot, Conan Doyle needed some identical copies of something where a jewel could be hidden. These copies need to be destroyed, in order to reveal the jewel. If you decide on using busts in 1904 for an American and British audience, Napoleon is an ideal candidate: notorious, but not venerated. Imagine what a scandal it might have been if busts of the late Queen Victoria or of Abraham Lincoln had been smashed.