Show an excerpt from a meta box?

If you just want to show, then you can use the custom_html field. This field can show any content in your meta box. You just need to pass $post->post_excerpt to its std attribute, like this:

global $post;

// Define meta box
...
'fields' => array(
    array(
        'type' => 'custom_html',
        'std'  => $post->post_excerpt,
    ),
)