Nice debate.

1-I have seen a 1" threaded fork used with a 1" threadless headset. The owner replaced the threaded-type headset with a 1" threadless headset. He gained just enough steerer length (in reduced stack height) to clamp on a stem, and cut off a quill stem bolt to use in the steerer (cleaned up the threads after cutting it off). He found an older 1" clamp stem; used it "slammed." My guess is that he's probably still got 1cm or so "space" under that stem cap. His only flexibility on sizing would be stem choice. I reminded him to re-tighten the stem on occasion, as I felt the clamping would eventually crush the threads a bit. Haven't seen him since, but I'm pretty sure he's still riding the bike (Kestrel 200SCi) I haven't heard that he's pranged the bike and broken himself or anything else. This is not my recommendation, I just saw it done. It's a cajone-driven approach.

1a-Move the approach above to 1.125" and let the shim give you the height you need to clamp on a stem. I've not seen it done, but I can see the logic in it. I'd guess the shim will give a less positive grip on those threads, though, as the shim is not designed to reduce circumference on tightening, like the stem clamp is. I'd probably cut a slot in the shim, were I to attempt such a thing. I'd let my brother test ride it first.

2-I have seen a 1" threaded fork swapped out for a Ritchey 1" threadless fork, headset swap, of course, and it worked very well. He had to use a 1 to 1 1/8 shim, as he wanted a more modern stem. The bike rides orgasmically. (Another Kestrel 200SCi)

3-I've seen many applications of the quill stem adapter. The older ones have a 1" steerer, the newer ones are almost all 1.125" They look "ok" and work fine. It's subjective, appearance, wise. At NAHBS for several years, the builders used threadless forks on finely-made steel lugged frames. I was not a fan of the look, but function over form, as it was.

Anything can be done, tried, and many things work to different levels of success. Ignorance is bliss. I somewhat agree with Kontact, in principle I'd refrain from clamping directly to the threads of a 1" threaded steerer, but I've seen it work, so go figure. If I were to rescue another Kestrel 200SCi from a landfill, I'd consider the options above, but try to figure out some way to test it. I probably would not try it on something like a Y-Foil.

A local bike shop near me still says you can't upgrade any 7-sp or less frame to 8/9/10, while my '89 Centurion is running DA9000.