Mount a router on the arm, put some rollers on the bed, and it could be a good surface planer for flattening large rough or cupped boards. Mostly I've seen jigs where the router is moved down the length of the board on a couple of rails, but here you'd just have to slide the board along on the rollers and move the router back and forth. You'd need some kind of mount for the board so that the plane you're routing stays the same.