Thanks for the suggestions!
I tried a modified version of the fishing approach, using the main halyard (as I don't have a spin halyard rigged). The disadvantage is that the main halyard runs up the aft edge of the mast whereas the shackle is stuck on the forward edge right at the peak. I taped a boathook to the main halyard with a band of tape just above the midpoint of the boathook and another band of tape near the end, this allowed me to hoist the boathook vertically, hook end up and able to extend above the masthead. I attached a spare sheet to the end of the main halyard for a tail/downhaul/control line. Then I led the halyard out around the shrouds -
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I wasn't able to catch it with this rig however. So next I borrowed a large magnet from the marina (they use it to fish up lost keys and such around the docks) and taped it to the hook. I got the magnet right up against the shackle but it wouldn't stick strong enough to pull it down.
One more thing I'm going to try before I give up and obtain or make a bosun's chair to go up myself (or better yet, find a lightweight person to go up while I crank the winch) - that is to attach a telescoping fishing pole to the boathook. I figure I'll have better control with the boathook only 1/2 way up and can grab the shackle with a fishhook.