Hi James!
So sorry to hear you've run into problems with the leather on your Brooks saddle cracking 'round the rivets.
In my experience, this can happen when the leather repeatedly gets locally soaked, then dries out too rapidly, causing a loss of essential oils in the leather. Given where you're touring (what fantastic photos on your website -- wow!), I could see something like that happening in the course of regular events. Periodic light coats of Proofide should help in the future, but for now, I think you may have to take more extreme measures.
I managed to field-repair a saddle in similar condition belonging to a friend while we were on-tour. I drilled out the rivet and replaced it with a dome-headed carriage bolt with a nut on the backside. the carriage bolt head allowed me to clamp the leather to the rear flange over a larger area when the leather was too far gone to stand pounding in an oversized Brooks rivet (which was also unavailable at the time/place). I then coated the leather on the worst of one side with the beta-cyanoacrylate (super glue) I always carry and applied a small duct tape overlay. The repair worked well. When the other side advanced to a similar state, we were near a larger town where we could purchase Shoe Goo, intended to repair shoe soles. A similar repair to that corner using Shoe Goo in place of super glue did even better, required no duct tape, and the saddle carried on for a good five years After until my friend tired of touring and sold the bike with the repaired saddle. Yes, it was a bit unsightly, but the repairs held and gave no further trouble.
Perhaps something there will prove helpful to you before the leather chunks out entirely in that corner.
Brooks offer a wide range of replacement saddle hardware, but no replacement leather covers, as I recall.
Very best wishes on your journey; may all good fortune go with you.
Best,
Dan.
