In this thread, including the co-author as a custom taxonomy in wp_query worked: http://wordpress.org/support/topic/plugin-co-authors-plus-wp_query-not-working-as-it-should?replies=9
Probably as a result of a later update, that solution no longer works, but it also isn't needed, because the original problem (where a wp_query by author or author_name listed only the posts on which the person in question was the PRIMARY author) has been solved. Hooraaaay! 'author_name' => $user_login totally works in wp_query!
Here's the weird thing:
Individually, my author_name parameter works.
My tax_query parameter works.
My meta_query parameter works.
In combination, my tax_query and my meta_query parameters work when both specified;
my meta_query and author_name parameters work when both specified.
But when an author_name AND a tax_query are both specified, I get an empty set, even when there should be posts!
It has just occurred to me how profoundly unlikely it is for anyone else to have run across this issue, solved it, and currently be reading the support forum...but it can't hurt to ask.
However, barring the impossible fantasy of somebody having that very specific answer, it would also be helpful to know if querying the author as a taxonomy should still work, because that'd give me another combination to try.