Good ideas here, thanks all.
I don't think it's hollow, so I was thinking along the same lines as this, but I hadn't considered a thumb screw, so was worrying about drilling for the tiller pin. This sounds like a great solution.
Failing that find a piece of tube that will slip over the outside. Steel can be painted fancy, brass can be polished. You can secure the extension with a thumbs crew as it will be easy to get somewhere suitable drill and tap.