Okay, I think I have a solution: Bilenky Cycle Works in Philadelphia can braze on a new threaded steerer in place of the original threadless one for $125, which strikes me as very reasonable. The new steerer will have rolled threads. More about that in a moment. I assume that the powder coat on the fork crown and surrounding area will be toast after the work is done. It wouldn't be hard to have the whole fork refinished--there's a powder coat outfit near me that seems to do excellent work on bike frames--but knowing me I won't bother. I specialize in cosmetically imperfect bikes.

The Yellow Jersey shop in Madison would be willing to thread my threadless steerer for $60 per centimeter. Figuring a headset with a 40mm stack height, that's in the neighborhood of $250. Steerer replacement wins big in terms of cost, compared to cutting threads.

Also, based on my correspondence with Steve at Bilenky and Andrew at Yellow Jersey, I gleaned the following information, although it's still not as definitive as I would like it to be.

Andrew seems to say that cut threads are indeed smaller than rolled threads, and don't engage as well with the headset threading:

"I have filled in cut thread and recut where the material was intended for rolling or AH [not sure what AH is JV] and the cut thread engagement was poor. On our fine-thread bicycle formats, a few thousandths matters.

If it were a one inch I'd be more positive about it. It's a better design and we have better piloted tooling.
For 1-1/8x26 the format is ridiculously fine thread for the diameter and tooling isn't quite as positive for new thread centering.
The threadless column material is on the small side for a perfect cut thread. A good caliper and two forks will show that."

I guess all that makes sense. Rolled threads on spokes--which is the rolled thread that I think most of us are familar with--are obviously larger in diameter than the spoke itself. But the fine threads on a threaded steerer are so small in proportion to its 1" or 1 1/8" diameter that it's probably not easy to measure (hence Andrew's comment about a few thousandths mattering.) It would mostly be reflected in a somewhat sloppier fit of the headset parts.

In practice that might not matter much. If threading the threadless steerer was a lot cheaper, I might be tempted to take that route. But since a new steerer with rolled threads (per Steve at Bilenky) is half the price and gives a better end result, that seems like the obvious way to go.