I'd assume the manufacturing process would be as follows..
Square taper is forged into a blank....
Do you think you could form this using a taper from a bb parallel turned behind taper to fit in a hydraulic press (in an adapter with suitable hole), press this into blank machined with hole dia (TBD), leave taper in place, mount in lathe on parallel section of bb, cut threads remove taper using crank extractor / heat, remount to taper (JIC any distortion to release/relieve from forging) then as above?
Very possibly underestimating the forging forces required??