I did not see this thread back in January, my folder is not a Dahon so can't elaborate on that. But I can say I am using a Sram Dual Drive on my Airnimal. Someone on this forum a couple years ago suggested that the Sturmey Archer 3 speed bar end shifter will index the Dual Drive quite well. I bought the bar end shifter and find it works great. Second gear is direct drive, third gear is an over drive, and first gear is a low range for lower hill climbing gears. It takes 135mm dropout spacing, I do not know what you have.

A friend of mine uses bull horn bars with bar ends on his full size touring bike, he is quite happy with adequate hand positions for loaded touring.

First photo is of the Dual Drive, second photo shows the Sturmey Archer bar end shifter close to the camera, the Shimano BS64 eight speed bar end shifter is farther from the camera and out of focus. I use drop bars.

About a year ago I saw news that Sram was taking the Dual Drive out of production. It still is available on Amazon and others in the retail pipeline, but can't say how much longer they will be around. I bought a couple spare click boxes and a couple extra shift rods just in case these become scarce later. I have not damaged either the click box or shift rod, but they are somewhat vulnerable on the driveside so I wanted spares.