Actually now that I've looked at some more code, I think you can piggy back it on another query so you don't have to add one. To try that, create a new plugin using hook cache_ordered_forums and code like this:
Code:
$counter_select .= ', thread.dateline AS lastthreaddateline ';
$tachyjoin .= ' LEFT JOIN ' . TABLE_PREFIX . 'thread AS thread ON(lastthreadid = thread.threadid)';
and then you should be able to use $forum['lastthreaddateline'] in your plugin (but I haven't tried it at all).
Edit: ...but I'm not sure if that's right now that I think about it. I don't know if the lastthreadid is the last thread created or the last thread that someone posted in.