Thanks to Bill Laine at Wallingford for the magic tip. The nose bolt on a Brooks isn’t held in by anything but tension. A little light tapping and prying had the whole thing slide apart.
New rivets, a couple hammers, a nail-set and a chisel, and the saddle seems functional again! The chisel split the hollow part of the rivet, and the nail-set peened the pieces over firmly. At the end, I went at the rivets with the wedge-part of the smaller hammer.
I thought I’d start with two across from each other, but it worked best to start with the one to the right of a bag loop (second from the corner), then start working my way across. It took too much pull to line up the leather hole with the cantle hole for the rivet.
The last rivet I had to sort of lever into place with the scratch awl through the hole.
The corner rivets were the hardest by far to really set well, and I wouldn’t be surprised if they ease up a little from completely flush in the future.
The saddle looks a little more scuffed than it did to start with, and there are rivet-dents in the piece of wood I used to protect the marble top of my typesetting bench. After I noticed the scruffs and scrapes in the marble… I started out using the metal rail, but got kind of carried away.