either consider a vastly different frame.. Bernie Mikkelsen has a ride around bike to adjust lengths and angles to design a custom frame by riding an adjustable one 1st.

Or, you might adopt some Brompton parts.. their Saddle adapter pin clamps on top of a plain seatpost.
theirs or something else.. It offers a horizontal tube

then you clamp a saddle clip onto the SAP,or on the seat post ..

Brompton has also made a nice saddleclip, so It allows a vast variety of angles . stepless and mostly aluminum.