I am posting this in case it is useful to add to the general pool of knowledge of things to try in the event of a halyard that has flown the coop.
How it happened:
Main halyard was being used to run a radar reflector up the mast. Reflector was long cylindrical Plastimo model. Halyard shackle was attached to one end of the reflector and a bowline tied to the other end. The halyard was not attached to itself to form a closed loop. When the device was about mid mast the device fell apart at the glue joint at one end (the end attached to the shackle). It ran up the mast with about a foot and a half of halyard exposed, and the shackle and plastic end of the reflector jammed between the mast and shroud on the port side (first picture). We opted to put a slight tension on the main halyard and leave it in place with the idea that it might be easier to retrieve with a predictable and fixed location of the halyard.
The topping lift on this boat runs through a block in the mast and represents a line that can be brought up to the wayward halyard. My thought was to attach some kind of hook to the topping lift, with a lot of spare line added, to run up and snag the main halyard and then pull downward, maintaining some tension on the main halyard to keep the line "snagged" and under control until it could be reached. This method was ultimately successful but took a number of tries.
1. Purchased various pieces of hardware that looked like suitable hooks, with turnbuckles and the like to use to weigh them down or attach them to the line, and purchased 50 feet of light (red) line to tie to the shackle in the topping lift and use to haul down on the hook.
2. Padded the boom with PFDs as a precaution and untied the topping lift (boomvang on this boat supports the boom and keeps it off the deck though).
3. Attached hook apparatus and spare line to the topping lift shackle.
4. Began fishing. By manipulating the angle of the red line while hauling up it was relatively easy to get the hook right in the vicinity of the exposed main halyard. Problem is the hook was usually oriented in the wrong direction, facing outwards away from the mast etc. Some way of controlling the orientation of the hook, either through weight or a attaching a small additional control line, might improve the success of this method.
5. During the course of a number of attempts, as I suspected might happen, the main halyard shackle and the piece of plastic it was attached to got jarred loose from their position wedged under the shroud. The remaining exposed halyard promptly ran up into the mast leaving the shackle and the knot it was attached to and the circular piece of plastic exposed at the top of the mast. The knot and shackle appeared to prevent the whole assembly from disappearing into the mast even if the piece of plastic was not present.
6. One final adjustment was made to the hook to account for this new situation: instead of hanging down from the topping lift shackle, the topping lift line was wound around the hook so that the hook rode higher than the knot and shackle it was attached to and could reach several inches higher than the shackle. (second picture) With the new configuration, the hook managed to snag the large plastic piece very shortly after being run aloft.
7. With the end of the halyard "hooked", we hauled it down with counter tension being maintained on the main halyard to keep things under control. As you can see in the last picture, the hook is around the plastic but could easily have let go without tension. The halyard was hauled down until it could be reached by hand and secured.
-Having the halyard looped to itself could have avoided the situation. Even with disintegration of the radar reflector, the line would never have been lost.
-It's very helpful to have an observer with binoculars standing at a distance in the right spot to tell you if your hook (or noose or "grabber" depending on method) is in a good location or in the right orientation.
-Some way of controlling or influencing the orientation of your hook or grabber would probably greatly enhance the effectiveness of this method.
-The specific circumstances influence the method chosen. I was encouraged to use a hook because of the exposed piece of halyard with a bit of daylight between the line and the mast and the fact that the exposed halyard was trapped and fixed in place.