When I built my first bike I had this very same idea. I even went as far as to bolt on a 44 and 48 tooth sprocket at the same time, sandwiched. Of course at the time I knew very little about motorized bikes. I ran the bike on the 48T for a while with a regular fixed tensioner just for break-in, plus I need the bigger sprocket because there are a lot of hills in my part of town. Later I got a spring tensioner hoping to prove that the bike would run just fine while using it so that I could come up with some kind of derailleur in the future. I worked OK for a while, but I ran into problems with chain length. When manually switching between the 48 and 44 tooth sprockets, my tensioner ran out of travel and the bottom and top halves of the chain were almost touching where the tensioner was pushing up (barely pushing). Due to a misalignment of my back wheel after I re-installed it following some maintenance I did experience one unexpected "shift". The bike sounded like a car or motorcycle shifting gears as you might expect. it accelerated briefly and then the chain jumped all the way off, so I stopped. The kicker came when (and this is important) I was coasting down a hill, I let off the clutch to accelerate for the next hill and I didn't time it right. The chain was still slack at the top because the tensioner spring had been pushed down as the wheel drove the chain rather than the engine driving the chain. When I let off the clutch, the slack chain slapped into the engine sprocket in such a way that it skipped a link and jammed in the engine. My engine stopped immediately and the back wheel locked up. I didn't crash, in fact it was remarkably smooth and controllable sliding on the tire at 30 mph. But I guess my point was that any derailleur based shifting system would be subject to the same chain slack problem that I experienced. That being said, I think that it would work with a freewheel on the left side, because a freewheel would not allow the chain to be pulled by the wheel.
Also, I hate messing with rag-joints, so both sprockets are still on my bike.