Chain moves side to side with gear changes, and adding extra chain tension with an extra drive sprocket could make the deraileur angry. Also, chain-tube geometry varies a lot. Not impossible, just more complicated than it might seem.

I ride an all-original 1970s steel-frame road bike, which seems like a prime candidate for this sort of product. I'm certain no sprocket-based design would fit it. I'd buy one of these if there was any way to lock it up.