Thanks to Spambait11 and Brakemeister for your suggestions.
Prior to posting this thread, I looked that the Gaerlan web site. He has a couple of nice 20 in. forks available but they have 1 in. steerers, not 1 1/8 in.
The RANS fork is a good idea. I did not come across it while searching the first time, so thanks for the link. The fact that it is aluminum is a little intimidating, but I'm open to the idea if I can get assurances regarding strength. Again, assuming it's strong enough, the rake does not bother me: indeed, I prefer it. Finally, since it is alloy, it should not be too hard to cut additional threads, assuming I can find the right 1 1/8 in./25 mm die.
I couldn't find the 100mm fork on the Dahon web site. I looked in Accessories and on the Swiss distributor's web site (in German ...).
But how about the idea of a threadless 1 1/8 in. fork? There are quire a few 20 in. forks available in the threadless format (although will admit that most of them are chunky BMX forks that don't interest me).
My bike shop professional experience pre-dates threadless headsets by about 5 or 10 years, and the two bikes that I have with threadless headsets ... well, let's just say I have never quite got the trick of adjusting them. Moreover, I don't even know the terminology.
My experience with threadless headsets is that the stem acts as the lock nut. Of course, there is no stem on a Dahon, and I don't fancy the thought of trying keep a headset adjusted by somehow fine-tuning the Dahon handlepost.
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.
Any further thoughts? Thanks again.