I can't use EITHER half of an ISO threadless headset in a JIS frame without modification, right? I'll have to have the head tube reamed out to use either of the cups from an ISO set, right?

Yes, I do agree that I can use the existing bottom half if I can't come up with a new crown race. It's the top half parts that I'm having trouble finding. Which was the genesis of my idea to just remove the threads from my existing top race. But I understand the concept of the compression ring spreading the preload to the bearing, requiring a pretty precise taper in that top race.