Traditional Dutch bikes have rather low angled head and seat posts,
but they are not lightweight frames.
[OTOH, they have been serviceable for a couple generations]

lightweight diamond sport riding frames have a steeper seat tube..

One of the pieces Clever Cycles, PDX actual NL bike importer,
(they will sell and ship one)
and Brompton folding bike dealer,
say they do is to add a Brompton part, Saddle Adapter Pin atop a standard
stepped down [7/8" top] seat post.. the SAP provides a horizontal tube also 22.2..

then the saddle clip is attached to the SAP,, which , pointing backwards,
gives you a couple inches..

Tall frame the angle change will be less, obviously, than a shorter frame.