I've seen one or two somewhere. You need a tensioner that will take up enough chain like a Melvin from Paul, or just an ordinary derailleur with its limit settings on your gear, which means you also need a derailleur frame. The ratios available are really big shifts but still don't have a lot of range, for an effect about the same as a 3-speed hub. It's waaaay less range than a typical MTB system, but obviously way more than a SS!