Thought I’d burn the installer on a DVD and try again. After booting from the DVD, this is all it does.
This is uncharacteristically painful for Apple, but it’s also a brand new install process, and I’m trying to do something unusual (install to USB stick). Usually you need to boot from a separate medium (DVD, USB drive) to install an OS onto another medium (your main hard disk). I’m surprised this install-from-App-Store process works at all.