maybe a small spinlock at the top, but then you'd have to run some sort of control lines down the mast (to open/close the spinlock).
The spinlock would not withstand the mainsail downhaul loads very well. I finally gave up on those on the spinnaker halyard after two of them exploded on me (fortunately right at the finish line both times). They work great for forces induced directly by hand but not behind something that gets loaded up with additional loads.
