Thought a small bit of rather thin, cable run out through the top of the seatpost,
a tube, stepped down at the top, they are open out the top..
looping around the saddle rails, and back down,
with both ends held by the Brompton rubber bung , in the bottom of the seatpost,
would work, and be un obtrusive. likely just hear the cable rattling in the tube as you go.
... were you to have an anchor for the cable on the frame,
coming out the bottom, it could also be stretched tight ,
Made so, when you pull the post up to your desired saddle height.
Badda Bing !
