jaredatch metaboxes on video post formats

You can find out the post format using get_post_format(). So try adding your textarea custom field in a custom metabox library file, and then do the following:

if( 'video' == get_post_format( $post->ID ) ) {
    include_once( get_stylesheet_directory_uri() . '/path-to-cmb-library/video-metabox-file.php' );
}

This assumes that you’re in The Loop or that you have a WordPress Post object in $post.