Not a very accurate title, but ...

So far as I can see, you can add only one user at a time to Zenphoto. Is there any way to suck in a file (of user IDs, email addresses, ... whatever) in some kind of canonical format and have them added?

This is mostly for the purposes of providing restricted access to some albums based on user ID or user ID/password.

In particular, I'd love to hand Zenphoto something like a list of email addresses (e.g., from a Mailman list) and, say, default passwords, and have Zenphoto add those as users.

The only alternative for me appears to be to use a "group" ID and password -- which is mostly okay. But being able to manage a (changing) set IDs and passwords would be ideal.

Any support for this?

There is no interface but you can certainly do that via the object model by code.
http://www.zenphoto.org/documentation/classes/Zenphoto_Authority.html
http://www.zenphoto.org/documentation/classes/Zenphoto_Administrator.html

Yeah, but I don't think I'll go in that direction.

What I'm trying to do is to set up a web site (and associated applications) that in the near future I can turn over to someone who has few, if any, technical skills -- but who would be able to maintain it. This means avoiding any special "local mods" or source changes, etc.

How realistic a goal this is remains to be seen, but so far I seem to be succeeding pretty well. In part this is because Zenphoto is quite easy to explain, understand, and use from the admin interface (with a little helpful "how to" documentation). In fact, it's largely Zenphoto that's allowed me to provide some really impressive capabilities for content management, but without anything beyond basic configuration of the CMS.

So one of my basic principles in all this is "Avoid any custom programming". In fact, "Avoid anything that even looks like programming". There's only so far you can go and conform to those principles :-). There are some trade-offs, but at this point it's coming out well.

Glad to hear that you get along well. To provide that is and was our intention. However, Zenphoto itself cannot and will never be able to provide solutions for everything out of the box. That is why there is a plugin system to extend what might be needed. So without custom coding cannot always be avoided.