Paul: you still need to have the axis aligned, otherwise you'll never get to the "poles" of the sphere. Although I can see the benefit of using the banjo with an existing clamping device, I think it'll be a faff to set it up every time, as the banjo moves freely in both directions (laterally as well as longitudinally). It's probably easier to make a new stand for the old gizmo, from plywood, with a central protrusion that fits snugly between the runners of the lathe bed.