For anyone interested, after getting advice here, I thought I would share the specifics of my experience going up the mast to replace my wind vane. I moor in a small lobster fishing cove with nary another pleasure boat in sight so the other options mentioned here of using larger winches from a neighboring yacht or pulling the mast over was not an option.
I recruited a limber, slim 25-year old, Oliver, who does slacklining (Slacklining entails balancing on a 2 to 5 centimeter wide piece of webbing made from synthetic fibers, which is rigged between two fixed points, often trees) and rock climbing on a regular basis. I didn't ask, but if he weighed 140 lbs, I'd be surprised. We used the main halyard and a bosun's chair, which I had stashed in my shed. A simple piece of wood with two holes on either end and and a rope through them (you can see the design of it in third pic). I wanted him to go up when it was calm, but given his experience and age he said "I am not afraid." He had climbed a J-24 while underway with no chair and initially said he didn't want to use it but agreed to in the end. The wind was probably 15-20 kts on the mooring with 1-2 ft waves. Not ideal. I said we could abort any time. He insisted we continue. We reviewed what needed to be done when he was up there, stuck a screwdriver and some tape in his pocket and tied the new vane to the bosuns chair so he could just pull it up when he needed it. That worked well so that it arrived at the top without damage.
He was able to shimmy the mast and I kept up with him on the winch-- we tested winching only and it would have been near impossible for me (68 and strong, but not that strong)-- I could move him only a few inches, so grateful he was a climber. The halyard went through a jam cleat and around the winch, so as he paused he was held in place. Once he was all the way up I put the halyard on a cleat. And then and only then did I dare take a picture (attached). He was able to replace the vane but not wrap the setscrew with tape because he needed both hands to do that. He had to have one arm around the mast so that he wouldn't swing.
Coming down, I took just one wrap on the winch and released the jam-- he mostly controlled his descent with his limber feet and legs (like a fireman's pole he said).
Recommendations: I think I would have needed a second person to use the spin halyard as backup. I still wish we'd had a back up. Oliver thought it would be good to shorten the line on the bosuns chair with knots so he could get closer to the top and the knots would give him a better grip on it. Definitely do it in calm weather no matter what he says!
The funny part was that this brave rock climbing, slack-lining youngster (see pic of slacklining over Dixville notch in NH) said that he was terrified up there with the boat pitching and his dramatic "oh sh*ts" testified to his fear. In the middle he asked if the mast was gonna break. I told him the people in this forum said it could be done. And given his experience he said "I think it says a lot that I was terrified up there and I loved it." He was also grateful he had the bosun's chair.
As payment, he wants me to take him out to Jewell island in Casco Bay, Maine where they have two ten-story WWII observation towers. He wants to run a slack line between the two of them and walk it...