Mark, and CRs:
I ran into the same difficulty of trying to retrofit a Brooks Swift onto one of my more modern bikes; I couldn't move it far back enough either.
I can provide the following supplemental info:
1. The entire "cut" of a Brooks saddle (the front to back taper) is shifted forward relative to a plastic saddle. This is to ensure that your butt bones sit on leather instead of the rear support plate (cantle plate?).
2. The rail design of the Brooks has a seatpost-mating portion which is shorter and shifted to the rear (which also pushes the saddle forward). I was told by Brooks that this is done so that the frame structure doesn't collapse or bend inwards due to the pressure of the front nose screw supporting the rest of the leather structure under tension.
3. I compared a Brooks Swift against a couple of Selle Italia plastic saddles: I placed the modern saddle on top of the Swift so that their front-to back tapers were matched when viewed from the top. I found that to equalize the front to back placement (I looked at the rails from the side and compared the front of the clamp areas) I would need a seat tube angle which was about one inch (!!!) less steep. This would change my frameset design from ST 60 cm ct x 74.0 ST angle x 57.5 TT to something like 61 x 71.5 x 60. This makes a frameset which is heavier, more flexible, and obviously designed to fit the Brooks to a near-exclusion of anything else (unless you used a no-offset seatpost with a modern seat.)
4.To quote my response from Brooks' Nick Sanders (on 3/12/02) regarding my similar complaint ("Help! I tried to ride a Brooks but couldn't!) I received the following suggestions:
Companies such as X-Lite ( http://www.x-lite.com ) produce high quality seatposts in sizes that fit both road and mountain bikes with a layback of around 27.0mm.
Other companies to look at may be RaceFace ( http://www.raceface.com ) and USE (Ultimate Sports Engineering - http://www.use1.com ). There are also many other manufacturers that do layback designs (some with a smaller price tag). Your local bike shop should be able to help you with this.
5. Bill Laine at Wallingford Bike (http://www.wallbike.com/) was also investigating some other high-setback seatposts. I recall that he emailed me infor about one modern non-carbon Vitus post that didn't look too bad.
6. I've heard (but not verified) that some of the (now-defunct) Ideale leather saddles (#90?) did allow a bit more rear setback.
However, I don't know of anything within the CR timeframe other than finding a vintage frameset designed to accomodate the Brooks (and likely a more forward riding position as well, I suspect).
Unfortunately, I'm still riding plastic because I haven't found a solution to my setback problems, either.
So, this bring up an interesting CR-related issue: Was the introduction of the Cinelli-Unicanitor saddle the most relevant issue in leading to modern frame design?
Andrew Gillis (rainy in Long Beach, CA)