you could use an onone
just an allen key to move the jockey wheel and its sprung so itl take up the chain slack
ive seen a few trials riders using 2 sprockets at the back , the usual large tooth for trials and a smaller to get to where they want to ride quicker than walking pace . push the tensioner and move the chain over by hand then readjust the jockey wheel . they set them up so the chainline is best for the larger sprocket as they put more strain through it
there is a tooth ratio that keeps the chain under the same tension in 2 chainring/rear sprocket configurations . solves the chainline issue as its a matter of moving the chain further in or out while keeping it parallel
but it gets to the point where you have to start wondering if it would be better to just use a mech / shifter and range of sprockets