I posted this recently but didn't get any response - I have a similar need but due to long thighs/legs rather than saddle rail constraints. Ignore the Thompsons, they are angled for setback but the clamp is still 'in line' so they don't offer much more than most standard posts and they're pricey too. Have a scoot on the FSA website, I managed to pick up a post with 40mm setback for £20 from wiggle and it still looks pretty sexy (can't recall the model).
I spent ages searching and FSA were the only company which did a good set-back without spending a small fortune.