Thanks, folks. I live in a small town. When my bike is used for shopping etc, the average ride between stops is seconds, not even minutes. Because the n'lock is so easy to lock and unlock, I literally ride across the street from the pharmacist to the library, twenty yards, and from there to the art shop, twenty yards, and from there to the bank, forty yards, and so on; I'm on the bike seconds at a time.
It would be a huge nuisance to take the saddle cover off and put it back on again at such short intervals. I think I'll just wear it out and replace it with something more durable, or resign myself to my beloved Brooks B73 getting wet and out of shape.
This is definitely a pain in the seating area.
I wonder if it would help against the rain to slather the seat with Brook's wax preparation (or Obenauf's), or really let the neatsfoat soak in (instead of just dipping the saddle in neatsfoot for twenty minutes like I did), or even try motor oil, as was used on Brooks in olden days, when even Sheldon was young.
...
I'm not surprised to discover that waxed cotton isn't permanently waterproof. I have a Berghaus Goretex/nylon mountain jacket, about a grand's worth, same jacket Chris Bonington went up Mount Everest in, and they gave me several new ones every time it started leaking until another hillwalker told me the secret: the Goretex lining is only half the protection, the other half is the nap on the nylon shell, so it helps to put the jacket through the laundry, including the hot air dryer, to bring up the nap. And it works. I also spray it with Nikwax every so often. Still, for that much money I'd expect it to work better, without contortions.
Not that the Brooks seat cover is cheap. I should have bought several B73 saddles when they were on sale for £49 at SJS...