I spent a couple years in Morocco, where it was very common to see doubles equipped with slings. Do not remember seeing anyone carry a slinged gun as you describe, but it's been a long time. But here's undoubtedly the funniest sling image I remember: Moroccan hunter, Moped mounted, shotgun slung diagonally across his back. Wife perched behind him, facing backwards, holding hunting dog in a big box on her lap. Wish I'd taken a picture!