Several early breech-loaders were modified muzzle-loaders, such as the British Snider-Enfield you mentioned, which was externally practically identical to the Enfield musket it was based on. It's not the only example, but it is the one that is always called to my mind, as they were used quite a bit, especially once the Martini came on the scene and the Brits started selling them off and issuing them to second-string units.
So I have no problem using muzzle-loaders as breech-loaders in games, as they were often so similar-looking as to not make a difference.