Toying with this concept at present, but not finding much out there online.

Tern/Dahon seat posts are 33.9mm external diameter, 27.2mm internal diameter, right? The Tern telescopic post’s upper section has a 27.2mm external diameter. So a 27.2mm dropper post should fit inside the top of a standard post then. Externally routed cable should be possible from there, following the rear brake and gear cable route to the handlebars. Anything missing from this so far?