It's possible with a very simple code edit:
Open the file: library/XenForo/Model/Session.php
Find:
PHP:
public function getSessionActivityQuickList(array $viewingUser, array $conditions = array(), array $forceInclude = null)
Shortly below that you'll see:
PHP:
$fetchOptions = array(
'join' => self::FETCH_USER,
'order' => 'view_date'
);
Change 'view_date' to 'username'.