Back around 1980, I think Suntour came out with a two speed system for BMX bikes. It used a two cog freewheel and included (IIRC) a derailer, cable and shifter. It was actually intended for BMX racing and the idea obviously never took off. This may be a update on the idea.

I'm guessing the derailer hanger is designed for the track-end style BMX dropouts. The freewheel also needs a BMX type flip flop hub with the smaller threaded side to accomodate sub-16t freewheels.