Adding Custom Meta Boxes


I'm trying to figure out how to add a custom meta box to admin pages other than the post, custom post type, and page write ones. I am doing those successfully using the instructions from WP -

I'd like to be able to addmeta boxes to any screen, to provide support info for my clients. For instance, On the Broken Link Checker page that lists all asite's bad links, I'd like to add a custom meta box. I'm hoping for a general solution that will work beyond that case, though, both in plug-ins and default WP pages.

The WP docs say to use this function:
add_meta_box( $id, $title, $callback, $post_type, $context, $priority, $callback_args );

Where I'm hung up is what to use for the fourth parameter, $post_type, if not 'post', 'page', or the custom post type slug.

Can anyone help?