Any question answered...

Where could I get a seat post custom built?

It may not be possible, but I'd like to see whether it's possible to have a seat post built that provides more layback than my current velo orange one (which gives pretty much the most layback commercially available seat posts can)

I don't mind if it's steel like the nitto s84 if strength is an issue
(whilst this nitto post has loads of layback at 40mm, the clamp is wider than teh VO one so i wouldn't give significantly more layback in reality)

I hate to tell you this, but there's only two solution;

• Get a modern saddle, or the Selle An Antromica which is identical to a B17 and does not need to be broken in (leather is laminated), just a different construction and have a long rail enough to set it further back;
  
  

• A new frame, despite the Longstaff being an excellent frame, it's not fit for purpose (i.e. you).