You get a seat post with a Head design offering more rearward Saddle position..

Alternate kludge that works : Plain Low cost seat Post . And Brompton's saddle adapter pin

(it offers a Horizontal tube on a typical seat post angle )

you then Use a Regular saddle clip It will be rotated so as to sit above the SAP. (I've done this on a Bike , myself)
adjustable set back distance that way..


On the delux end There is the Nitto Nickel Plated Chromoly seat post , Riv Bike sells those (only in 27.2mm)

Nitto Lugged seat post 27.2 x 250 - 11048