Thanks to all for the advice. We failed on our initial attempt as we didn't note that the (damned) fixed topping lift was wrapped around the main halyard we were using to hoist the chair. So I'll add "inspect the mast head with binoculars" to the to do list for others in the future. There was just two of us and it got too late with us too tired to continue.
In addition, if you lack self-tailing winches, having a third person to tail, especially if you are using a wimpy little Lewmar 7 on the cabin top like we were, was a huge help. We switched off twice and it was no problem.
We had the mast secured by the fixed topping lift to the boom end, and the aft traveler's main sheet taught. We also led the jib halyard around the spreaders aft and secured it by clipping the wire halyard to an anchor line, running it as far aft as possible, through a block on the toe rail forward to the primary winch. We winched it snug and cleated it off. Once the climber go to the spreaders, he tied off a 1/2 inch line and we ran that to the other primary in a similar fashion and cranked it snug. Any shackles we used were tapped and had a back-up shackle to boot. When he got to the top he asked us to snug things up a little more so we did so. We used the main halyard to haul him up with the spinnaker halyard as a safety line. With two people on deck it was easy to ensure that the safety line was always taught. No doubt the other days practice helped a lot too. Another piece of advice might be to try your approach out with a bucket of water or something before sending a person up. Communication can be challenging if it is even slightly windy, so it pays to work out some signals beforehand.
When he was at the top we loosened the turnbuckle below, but ultimately had to detach it to give him enough space in the mast head to get the pin out up top. He then pulled up the new one on a line we had provided him, pinned it in and then let to old stay down on that line. He was up and down in about 15 minutes. We had set up a plumb bob to the gooseneck and marked the deck with tape to allow us to ballpark the rake. We then rough cut the stay, checked it then did the final cut with a 32 tooth/inch hacksaw in a jib I had made up. Installing the stalok was a piece of cake. We snugged everything up, cleaned up and left. It took about an hour and a half from start to finish.