Hiding menu items dynamically
use theme_menu_link() to modify your menu links on the fly:


style="display:none"
class="invisible"


e.g.

function mytheme_menu_link(array $variables)
{
   if ($variables['element']['#href']=='temp-menu-link') {
       $variables['element']['#attributes']['class'][] = 'invisible';
   }
   return theme_menu_link($variables);
}


Security note: It'll still be possible to access the URL, thus a "state check" must be implemented on the receiving end if temporarily invisible also means 'not permitted to call'.