example_post_status.php

<?php
// No, Thanks. Direct file access forbidden.
! defined( 'ABSPATH' ) AND exit;

// INIT
add_action( 'after_setup_theme', array( 'unavailable_post_status', 'init' ) );

class unavailable_post_status extends wp_custom_post_status
{
	/**
	 * @access protected
	 * @var string
	 */
	static protected $instance;


	/**
	 * Creates a new instance. Called on 'after_setup_theme'.
	 * May be used to access class methods from outside.
	 *
	 * @return void
	 */
	static public function init()
	{
		null === self :: $instance and self :: $instance = new self;
		return self :: $instance;
	}


	public function __construct()
	{
		// Set your data here. Only "$post_status" is required.
		$this->post_status = 'unavailable';

		// The post types where you want to add the custom status. Allowed are string and array
		$this->post_type = 'post';

		// @see parent class: defaults inside add_post_status()
		$this->args = array();

		parent :: __construct();
	}
}

wp_custom_post_status.php

<?php
// No, Thanks. Direct file access forbidden.
! defined( 'ABSPATH' ) AND exit;
/*
Plugin Name: Custom Post Status
Plugin URI:  http://
Description: Adds a custom post status for posts, pages or custom post types
Author:      Franz Josef Kaiser
Version:     2012-06-14.1426
Author URI:  http://unserkaiser.com/
License:     MIT
 */



class wp_custom_post_status
{
	/**
	 * Name of the post status
	 * Must be lower case
	 * 
	 * @access protected
	 * @var string
	 */
	public $post_status;


	/**
	 * Post type (slug) where the post status should appear
	 * 
	 * @access protected
	 * @var string/array
	 */
	public $post_type = array( 'post', 'page' );


	/**
	 * Custom Args for the post status
	 * 
	 * @access protected
	 * @var string
	 */
	public $args;


	/**
	 * Construct
	 * @return void
	 */
	public function __construct()
	{
		#echo '<pre>'; print_r( $this ); echo '</pre>';
		// We need to have at least a post status name
		if ( ! isset( $this->post_status ) )
			return;

		add_action( 'init', array( $this, 'add_post_status' ), 0 );

		foreach ( array( 'post', 'post-new' ) as $hook )
			add_action( "admin_footer-{$hook}.php", array( $this,'extend_submitdiv_post_status' ) );
	}


	/**
	 * Add a new post status of "Unavailable"
	 * 
	 * @return void
	 */
	public function add_post_status()
	{
		global $wp_post_statuses;

		$defaults = array(
			 'label_count'               => false
			// defaults to FALSE
			,'hierarchical'              => false
			// defaults to FALSE
			,'public'                    => true
			// If NULL, then inherits "public"
			,'publicly_queryable'        => null

			// most important switch
			,'internal'                  => false
			// If NULL, inherits from "internal"
			,'exclude_from_search'       => null
			// If NULL, inherits from "internal"
			,'show_in_admin_all_list'    => null
			// If NULL, inherits from "internal"
			,'show_in_admin_status_list' => null

			// If NULL, will be set to FALSE
			,'protected'                 => null
			// If NULL, will be set to FALSE
			,'private'                   => null
			// not set by the core function - defaults to NULL
			,'show_in_admin_all'         => null
			// defaults to "post"
			,'capability_type'           => 'post'
			,'single_view_cap'           => null 
			// @internal use only - don't touch
			,'_builtin'                  => false
			,'_edit_link'                => 'post.php?post=%d'
		);

		// if FALSE, will take the 1st fn arg
		$defaults['label'] = __( 
		 	 ucwords( str_replace( 
		 	 	 array( '_', '-' )
				,array( ' ', ' ' )
		 	 	,$this->post_status
			 ) )
		 	,'cps_textdomain' 
		 );

		// Care about counters:
		// If FALSE, will be set to array( $args->label, $args->label ), which is not desired
		$defaults['label_count'] = _n_noop( 
			 "{$defaults['label']} <span class='count'>(%s)</span>"
			,"{$defaults['label']} <span class='count'>(%s)</span>"
			,'cps_textdomain'
		);

		// Register the status: Merge Args with defaults
		register_post_status( 
			 $this->post_status
			,wp_parse_args( 
				 $this->args
				,$defaults 
			 )
		);
	}


	/**
	 * Adds post status to the "submitdiv" Meta Box and post type WP List Table screens
	 * 
	 * @return void
	 */
	public function extend_submitdiv_post_status()
	{
		// Abort if we're on the wrong post type, but only if we got a restriction
		if ( isset( $this->post_type ) )
		{
			global $post_type;
			if ( is_array( $this->post_type ) )
			{
				if ( in_array( $post_type, $this->post_type ) )
					return;
			}
			elseif ( $this->post_type !== $post_type )
			{
				return;
			}
		}

		// Our post status and post type objects
		global $wp_post_statuses, $post;

		// Get all non-builtin post status and add them as <option>
		$options = $display = '';
		foreach ( $wp_post_statuses as $status )
		{
			if ( ! $status->_builtin )
			{
				// Match against the current posts status
				$selected = selected( $post->post_status, $status->name, false );

				// If we one of our custom post status is selected, remember it
				$selected AND $display = $status->label;

				// Build the options
				$options .= "<option{$selected} value='{$status->name}'>{$status->label}</option>";
			}
		}
		?>
		<script type="text/javascript">
			jQuery( document ).ready( function($) 
			{
				<?php
				// Add the selected post status label to the "Status: [Name] (Edit)" 
				if ( ! empty( $display ) ) : 
				?>
					$( '#post-status-display' ).html( '<?php echo $display; ?>' )
				<?php 
				endif; 

				// Add the options to the <select> element
				?>
				$( '.edit-post-status' ).on( 'click', function()
				{
					var select = $( '#post-status-select' ).find( 'select' );
					$( select ).append( "<?php echo $options; ?>" );
				} );
			} );
		</script>
		<?php
	}
}