Hi Andre!
My, what a "thorny" (sorry!) problem; the requirements seem to preclude a ready solution. Still, let's look carefully at your requirements...
* Seatpost diameter 26.4mm -- If it isn't available in 26.4mm or smaller diameter, don't even mention it.
I looked to see if a 25.4 (used with a shim) might open up possibilities, but it doesn't seem to.
* Vertical space available between top of seat tube and lowest rail of Brooks B73 is c60mm.
Ah! This, partly because of the dual-rail configuration taking up some of the available vertical space. Andre, is there a single-railed version of your B73
that would suit? If so, it might then allow fitting of a sus-post of some sort, but I'm not sure the net effect would put you ahead of where you are now in terms of comfort. Otherwise, I think you'll have to decide between the dual-railed B73 (with its own suspension) /or/ a suspension post and single-railed saddle. Unfortunately, telescopic sus-posts aren't as responsive as parallelogram models due to stiction, but I don't see a parallelogram model as possible given your other requirements. Para' units all have layback and the movement is down-and-back rather than vertical.
* Because there are two rails, a very special adapter is required. This fits between the two rails of the saddle to each side and then the microadjustor at the top of a modern seat post fits around the rails, fooled into believing it is dealing with one rail. You just need longer adjustment bolts. I have such an adapter, kindly given to me by Julian (our Julk). Any seat post I buy must work with this adapter because there isn't a huge choice of adaptors, and I'm in Ireland where delivery is expensive.
I think you might run into trouble fitting this "clamp sandwich" adapter to a zero-layback 'post. As I recall, it was originally designed by Joe Breeze to fit an SR La Prade single-bolt 'post.
* Zero layback is essential. I already lost several millimeters of height on the handlebars when I changed to n'lock stem and bars, and don't want to lean forward further; there is zero latitude for anything but a zero layback seat post.
<nods> I understand your requirements here, but I also kinda quail a little when saddle position is used to accommodate reach rather than position behind the bottom bracket. Andre, at one time, there were adapters (SJS Cycles had them) to allow *more* layback on zero layback 'posts. These could be reversed on layback posts to create a virtual zero-layback configuration, thus meeting your needs with a more conventional option, but..why? There are more elegant solutions, like getting a zero-'back post that suits to begin with.
* Suspension seat post, if it fits the vertical height limit of 60mm to the lowest rail, preferred; otherwise good quality solid with twin screw micro adjustment preferred. The Airwings suspended posts are fitted as optional equipment by the makers of my bike but the smallest of them wants 98mm of vertical space.
I just can't think of an *effective* sus-post that will fit this space with no layback. I've attached a photo of the hinged elastomer puck-type sus-post I have for the captain's saddle on my tandem. It basically tilts, doesn't do much except quell road buzz, and fits in minimal space (necessary to accommodate the adjustable stoker's stem I made)...but has layback.
* Prefer black but will take silver if I have to.
Color will be secondary to function here, methinks.
Andre, looking at your shortlist of options, I think you've already found the most likely candidates to fit your needs. I would caution, however, that you may need to mill Julian's adapter to work with either (the fore-aft bolt positioning might not clear the "sandwich" adapter). Andre, it really isn't that hard to build the 'post you want. A local framebuilder or machine shop could whomp up something pretty quickly or mill a better, more solid and secure alternative to the Brooks clamp for a straight 'post. I've attached a couple more photos showing an alternative I made to address my Folder's unique needs that is both light and solid as a rock.
EDIT: Thinking further, would your present Brooks clamp do if the clamp halves were simply brazed or welded in your preferred position/angle so it wouldn't shift? If so, it would be a matter of seconds to accomplish and cost less than USD$10 at any weldor's.
All the best,
Dan.
