Congrats dfisher on getting the spinnaker halyard ran. It's not easy doing that from the top of the mast.
A few months ago I ran a new halyard and new coax. Also installed a new anchor light, steaming light, antenna and replaced the wiring for the anchor light and steaming light. This all while the mast was up. It would have been easier to drop the mast to do this much work, but doing it myself at the dock was free.
On my mast ('84 H31) there are four sheaves at the top and four at the bottom. There was no cover at the top which made top access easy. Unfortunately, as a result a bird had dropped in there and died (I'm not sure which order) and that had to be fished out (many bones) before I could get any messenger down.
I put the messenger down for the halyard from the top. One funny thing was, in order to get the bird out, I also brought a water hose up to to the top of the mast and squirted it down in the mast. I was a little surprised city pressure was strong enough to push water that 40+ feet up in the air.
Anyways, on the messenger I tied a small metallic rod and then had a friend fish it out at the bottom through one of the sheaves with a retractable magnet.
Later I ended up just cutting a hole in the mast for access to all the wiring (and remove said dead bird). I'm planning to have a custom cover flange mounted to cover the hole for strength and such, but haven't done that yet.