Who said anything about Quill Seat posts?
OP asks
So, which Brooks saddle sorta feels like B17 on your butt, but with a full range setback? (narrower is OK, preferred) It certainly isn't the Pro, there is a pronounced hump in the Pro, and the setback is terrible. Swallow? Cambium?
You asked
I don't think they make them for quill seatposts, do they? I mean I've read of people making their own with the bolt from a nitto technomic or something, but I don't think there's a non-DIY solution.
If not into fabrication hire someone to do It.
Brompton's SAP* and Penta (saddle) Clip combine nicely to use a straight Seat Post that is easy to Modify , as described .
and even easier if you hire the job out to a Machine shop.
*Saddle Adapter Pin, It clamps on top of the seat post and is a Horizontal tube that you clamp a saddle clip onto
and rotate the clip, so the saddle rails are above the SAP
and where along the Horizontal 22.2 tube you place the clip is your setback adjustment.