I did see one air boat that had the air rudder setup so they could be closed to act as a thrust reverser. Not sure how he did it though. The article did not go into how he set them up and it was a long time ago. Maybe add a channel that moves a cavoer in place. Might not have left and right, but would slow or back the boat up a bit.
There are many ways to skin a cat, with enough cats we can figure out almost anything.
Note, no real cats were harmed in this thread.
Buzz.