No idea of wordpress, but
> there is: /usr/share/wordpress/wp-config.php:
> define('WP_CONTENT_DIR', '/var/lib/wordpress/wp-content');
>
> but that doesn't have any effect unless I define it in
> /etc/wordpress/config-www.domain.tld.conf too.
what happens when you define it _only_ in /etc/wordpress?
you should not need to touch stuff in /usr/ for configuration purposes per file hierarchy standard. If that's how it's usually done for wordpress, that's weird. I don't know wether there exists policy for packaging concerning modifying paths, but I'd find it reasonable to change this. However, if debian packages differ from upstream here, that would definitely necessitate documentation.
regards, arian
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