IGNORE: The problem was in my brain.
I've been tooling up a new site, doing some experimenting in MAMP on my desktop, and then reproducing my work in a DH install of Wordpress.
I'm building a child theme for my site, which involves a custom taxonomy and custom post type. I've got a taxonomy archive file that renders correctly on my desktop install, but ceases output midway through in my DH install. It's the same file. Permissions are OK (in any event, the PHP wouldn't get halfway through if it were a permissions issue). Not sure what else could have changed.
It's nothing exotic—I don't think I'm using any PHP dependent on one environment. Just ordinary Wordpress function calls.