Panels returns NULL from menu_execute_active_handler()

Is there any way to define its own constant?
Current:

/**
 * @name Menu status codes
 * @{
 * Status codes for menu callbacks.
 */

define('MENU_FOUND', 1);
define('MENU_NOT_FOUND', 2);
define('MENU_ACCESS_DENIED', 3);
define('MENU_SITE_OFFLINE', 4);

When you do in ./panels_node/panels_node.module

define('MENU_IGNORE_THEME', 0); // or any other number

/**
 * Pass through to the panels content editor.
 */
function panels_node_edit_content($node) {
...
  print theme('page', panels_edit($display, "node/$node->nid/panel_content", $content_types), FALSE);
}

Or just return this:

  return 0;

Most of the code which has broken menu callback, return just NULL and it's confusing.

Note: If you return 0, isset() will still return false, and page will not be rendered.

elseif (isset($return)) {

See: http://php.net/manual/en/function.isset.php

Raised a new ticket against 7.x core:
#940264: Define new menu status code to ignore page rendering in index.php

Please check the following patch.
For you it doesn't change anything, for me yes.
Because there is a difference between NULL and 0
Thank you.

Patch as I can see in API way:
http://drupal.org/node/224333#theme_page

function mymodule_page_callback() {
  // Assemble page output here...

  // Return page output for Drupal.
  return theme('page', $output);
}

It's the correct way to pass already rendered page with custom arguments?

Ignore patch #3, check this.
Sorry, I couldn't test it.

If you need to alter page attributes, you can use the following approach. This can be used to alter the page output for one of your module's page callbacks. For example, to have one of your module's page callbacks not display any sidebars:

function mymodule_page_callback() {
  // Assemble page output here...

  // Return page output for Drupal, hide blocks.
  return theme('page', $output, FALSE);
}

Source: http://drupal.org/node/224333#theme_page

Isn't exactly what we trying to achieve here?
"Return page output for Drupal, hide blocks"?

I'm not sure, I've to test it as well.
http://drupal.org/node/428744#comment-1617886

Tested in index.php:

    $return = menu_execute_active_handler();
+ $return = theme('page', 'blah blah', FALSE, FALSE);

It does duplicate.

This doesn't:

  $return = menu_execute_active_handler();
+  print theme('page', 'dupa jasia', FALSE, FALSE);
+  $return = 0;

So the patch #4 will work.

Return 0, because if there is assignment in index.php

$return = menu_execute_active_handler();

code expecting that function (menu handler) will return anything.
Zero, because we don't want to execute none of the block within index.php with following conditions: is_int($return), isset($return).
Zero, rather than NULL, because module that are invoked later know that page content has been rendered correctly.

Ah, sorry.
Commented in the patch.