Plain seat post in needed diameter, Kalloy makes them,
Brompton Saddle adapter pin and their Penta clip saddle clip.

SAP is horizontal when installed on a standard 7/8"OD top seat post,
the stepless, mostly aluminum,
180 degree adjustable clip, will hold the saddle over the horizontal tube .

Todd at Clever Cycles in Portland does this setback mod a lot for 'dutch bike' riding postures, on your typical UJB frame .

I agree , seat post insert depth cannot be too short.