Velo Orange mentions that the rails on their leather saddles are approx 10mm longer than Brooks' rails, but are they longer in the direction that will allow the saddle to be pushed back more?

Yes.

alternately, you can use the brooks and a longer-setback VO seatpost...

Or do what I did and design the seat post angle to accomodate a brooks with a straight post.

Or he could build his own leather saddle. Or he could buy a helicopter.

Frame is a SOMA Saga with a 73d seat tube angle. The Surly LHT has a seat tube angle that's 0.5d slacker, which would buy me another 0.5cm of offset. But, the LHT doesn't really fit me. Something with a 72d seat tube angle would be great, but finding an inexpensive commuter-friendly frameset with that geometry is tough. A custom frame would be even better, but there's the whole cost/price issue, and of course, potential geometry issues (combination of deep BB drop, slack seat tube angle, etc).