Anything analogue is not very reliable in FS2.
This is a really cool idea. The rotaries on a midi controller are digital using a 0-127 general midi range. However they aren't designed to exceed the max value without something in between the keyboard and the computer so the board inside the controller would likely stop sending a value once it reached 127. Not sure how to get around that but if anyone tries it I would be interested in doing the same.