I've often thought that if there were some sheaves in your mast cap, you could run the spinnaker halyard over the top of your mast and into the luff track on the back of the mast. There would still be a bit of exposed halyard, but far less than we currently have. There would probably be too much friction (because mainsail bolt rope, mainsail halyard, and spinnaker halyard would all be in the same track).