Only show metabox when date-value in other metabox is over?

Not sure about the linked class – it seems they collect metaboxes immediately and so there is no information regarding what post is being viewed.

In general though – yes it is possible. To add a metabox:

add_action( 'add_meta_boxes', 'myplugin_add_my_custom_box',10,2);

See the source code here. This add_meta_boxes hook passes two variables: the post type and the post object. You can use the post to get the post meta and then call add_meta_box where appropriate.

function myplugin_add_my_custom_box($post_type,$post){
     //Get event date as a timestamp
     $event_date = (int) get_post_meta($post->ID,'_wr_event_date',true);

     //Check date exists and is in the past. If not, return: don't add metabox.
     if( empty($event_date) || current_time('timestamp') < $event_date)
         return;

     add_meta_box( 
        'myplugin_sectionid',
        __( 'My Post Section Title', 'myplugin_textdomain' ),
        'myplugin_metabox_callback',
        'event' 
    );      
}

You’ll notice there is also a add_meta_boxes_{$post_type} hook – this is more efficient if you want to just add it to ‘event_cpt’ post types:

     add_action( 'add_meta_boxes_event_cpt', 'myplugin_add_my_custom_box',10,1);

In this case your callback only includes the $post as an argument!

Note: Avoid using php date/time functions: this will, by default, always have the timezone set to UTC. If you want the current date/time in the blogs timezone using something like current_time()