Replaced the front brakes yesterday and found this.
The crack wasn't even visible until the shoes were removed because it was the shoe that mounts closest to the fork on the pin. The crack was directly under the other shoe on the mounting pin.
Cleaned everything up, smoothed out the hub, lubed up the cam and pin, put on a nice set of Newfren shoes - which by the way are much heavier duty - and now have a front brake that actually works.