I don't know of one as such but I built a X-Air standard and modified my brake drum system so that I could use a single lever, I'm a wheelchair pilot and I fly in the right seat and wanting to leave the standard toe brakes as they are so I drilled a second hole and fitted a double cable on each side so I have my single lever and the toe brakes are standard assembly, you could do the same but mount it to a hand brake lever so you have both systems.
the threaded ends and extra brake cable I purchased from a push bike shop just so you know :)
I attached a picture of the hub I modified
cheers Pete ??
