Update: I got my mitts on the saddle, did everything that Knotted advised and put the old Brooks on my commuter and set on an evening ride last night. It split apart before I even went a block, so game over. A shame, but the split really was huge - right through the entire leather and right on the nose of the saddle. But thanks for your help Knotted!

OTOH I got some absolutely beautiful vintage Campag brakes from the Recyclery in San Rafael for $35... they're going on the fancy lightweight of mine