The chuck is located on the plain register, so provided it's not a sloppy fit then I suppose a flat on the thread won't matter. It depends on the lathe as to how you can achieve this. On the Boxford, for example, it's possible to reduce the thickness of the spindle bearing cover plate to expose a plain section of the spindle. You can then fix a split collar to the back of the chuck back plate which then clamps around the spindle. There was mention of this on the Boxford Users' Group recently.