A terrible idea, as is any other user-submitted content that isn't properly vetted by the moderation staff. This isn't Minecraft where players can restrict what others see. And as Sweet Brew said, those with bad intent will abuse the privilege.
Even allowing users to position things in-game has the potential to be abused. Thankfully, it takes too much cooperation to use player ponies for things like this, and individuals who try can be moderated.