Originally Posted by ETRTO 520
On the other hand, if someone makes a threadless headset with one or more (lateral) set screws on the top races, it would be theoretically possible to assemble the headset and adjust the fork, then attach the handlepost.
Hmm...
Don't know if you saw this while cruising the RANS site, but this
article talks about a handlebar modification with a threadless headset. Not only that, but they sell the components you need to make this modification. I assume that once you're able to lock the forks into the head tube then setting the handlebar stem is pretty straightforward? In any case, I think the clamp they use in the article matches what you're describing as "(lateral) set screws"?
Another thought might be to use the Diatech headset with a head tube reducer as posted
here on Gaerlan's site to reduce the head tube diameter from 1 1/8" to 1" - then you can use a threaded 1" fork. Unless, of course, the Dahon stem won't fit...
But if folding is not a priority to you, maybe you can just make your own handlebar stem/post?