Interesting post. Thanks for taking the time to figure this all out, document it, and share it.

I've been researching the possibility of adding more chain rings to my Dahon Mariner D8. I was contemplating just getting a size or two smaller chain ring to drop the gear-inch range for more low end to climb hills. I don't like going too fast on the bike, anyway, so the taller gears don't do me a whole lot of good. As long as I can get a few gears in my normal range, plus one or two higher, and still be able to drop down to get to climbing I'd be happy. However, if I added a smaller second chain ring I could open up the gear range a ton more than just a size change in the one chain ring.

I did find some FD adapters online that will allow mounting on the larger seat tubes on the folding bikes. I don't have any links right off hand, but there are ready-made adapters out there.

However, regardless of there being a "part" out there to buy that will allow the mounting to start with, the chain line and clearance of the FD are still tricky - and I haven't seen as good of a description of that anywhere yet that even gets close to this one. So good job and thanks for the explanation!