I think it might be easier, though, to simply use a breezeR adaptor for a double rail saddle and then you can use a micro adjusting seat post to get more setback on a brooks double rail saddle . . .
and so first you need a bike other than Brompton , or buy their 2 part, telescopic seat post,
then Buy another seat post to replace the upper portion..
BTW, Brompton Pentaclip is already a perfect step less micro angle adjusting saddle clip for Plain top seatposts.