Oh the parts are available (and I think Helix offering the HK spec is awesome), and fit fine on 24" and 20"/406. But skinny 349 as you recommend lightweight external gearing on the T-Line... I'm with you, sounds better to me, but will it "package" with the RD not scraping the ground? I think perhaps yes, if using the style RD with a third pulley that I saw pictured on the Dahon Curl D9, that cleverly takes up more chain slack without a longer cage. Maybe also some of the newer 1X RDs with I think an additional pivot on top or a dynamic B-screw pivot or something, I haven't messed with any of those yet.

I went 20" folder (no smaller) specifically to avoid needing an IGH for high enough top gear, as my 1st gen Dahon which I got for $10 because it only worked in 2nd gear, I finally got tired of walking up hills so pulled it off to check, it seemed to shift into 1 and 3, put it back on the bike, and the cones loosened up after riding 10 feet, multiple times. I goofed up something, probably with some retaining tab or washer, who knows, but I wasn't going to spend $100+ at Aaron's Bike Repair (IGH specialists) on a well-worn $10 bike.

So I'm with you Jipe, I'm singing the same tune, light external gearing on a T-Line would make a handy and carryable urban assault and travel tool, if it fits. Brompton's 4-speed external just doesn't have the range.