How can meta boxes be added to “unknown” custom posts?

Question

I’m not sure what you mean by “unknown custom posts”, but here is how you’d add a meta box to a number of post types:

$post_types = array(
    'post',
    'page',
    'my_custom_post_type',
);
foreach ($post_types as $post_type)
    add_meta_box('Intro', __('Intro'), 'my_meta_box', $post_type, 'normal', 'high');

If you would like to have different contexts and/or priorities, you have to do it in multiple calls.

If I misunderstood you, please enlighten me.

// Edit
Of course, you could do it the other way around, and exclude post types that you do not want to have this meta box.

$args = array(
    'public' => true,
);
if (! is_array($post_types = get_post_types($args)))
    $post_types = array();

unset($post_types['post']);
unset($post_types['attachment']);
unset($post_types['my_custom_post_type']);

if (count($post_types))
    foreach ($post_types as $post_type)
        add_meta_box('Intro', __('Intro'), 'my_meta_box', $post_type, 'normal', 'high');

If this isn’t what you’re looking for either, I’m afraid I have no idea what you would be satisfied with.

You don’t want to specify post types, but want some functionality bound to specific post types…