FWIW another option is to use the boat hook to haul the halyard down, put a sail tie around the boom several feet aft of the mast capturing the halyard underneath. This will pull the halyard/headboard down and aft of the mast while in an area you can (hopefully) reach. We do this when securing halyards for the night at anchor. It keeps them from slapping. Since you are dropping the mainsail completely it might work for you.