I seem to remember attaching my leading thread to the main halyard and drawing my jib halyard down the mast that way (ie hitching a lift)….then fishing out at the bottom………I had been going to do what you are suggesting but the space required to get a reasonable sized nut into the mast seemed prohibitive. Obviously the spin halyard will be a slightly different job.
As a total amateur I have removed spin rigging off both a Wayfarer and a Wanderer but both boats have had the halyards in the blocks at the base of the mast as you describe. All the best