$threadTitle = $this->_input->filterSingle('title', XenForo_Input::STRING);
$threadTitle = $title;
$threadTitle = $thread['title'];
The following code works to get the threadId:
$threadId = $this->_input->filterSingle('thread_id', XenForo_Input::UINT);
I suppose I could use a query to get the title based on the $threadId, but I'm hoping there's some code to get the thread title directly.