Hey, Patrick, good idea on the old sail.
The other poster suggested the head plate piece from another sail, but I think that too might twist. Maybe in additon to the headplate, a piece of line from headplate to top batten, would provide the twisting force. i.e. when you twist mast away from sail, the line tension would keed headplate lined up with the sail, not the mast, and it would unhook.
Another idea - install a ball-and-fork on front of mast (like old 5.2 style main halyard), at height best for the short sail. Still use hook & ring on backside of mast for regualr sail.