I have done this myself. In an advanced theme, try these two functions:
function page_title_error()
{
if ( isset($this->content['q_view']['url']) )
{
$this->content['title'] = '<a href="' . $this->content['q_view']['url'] . '">' . @$this->content['title'] . '</a>';
if ( @$this->content['q_view']['raw']['closedbyid'] !== null )
$this->content['title'] .= ' [closed]';
}
parent::page_title_error();
}
function q_item_title($q_item)
{
// display "closed" message in question list
$closed = @$q_item['raw']['closedbyid'] !== null;
$this->output(
'<DIV CLASS="qa-q-item-title">',
'<A HREF="'.$q_item['url'].'">'.$q_item['title'].'</A>',
($closed ? ' [closed] ' : ''),
'</DIV>'
);
}