Still no progress in my problem with custom tab in admin siteaccess.
I found in eznavigationpart.php that:
<i>
Will return the navigation part array if the identifier is valid, the default will be returned if the identifier is not valid </i>
But what is valid identifier?
later in the same file: <i>The list is cached in the global variable eZNavigationPartList.</i>
My custom identifier is in that variable, but doesn't work - tab has still 'inactive' style and variable $menu_item.navigationpart_identifier is set to 'ezcontentnavigationpart' which is default navigationpart identifier.