Here's how I did it--some of this code exists by default, some of it is changed. I've underlined the changes.
<xen:include template="ad_thread_list_below_stickies" />
<xen:hook name="thread_list_threads">
<xen:foreach loop="$threads" value="$thread" i="$i">
<xen:if is="in_array({$i}, array(9, 15))">
<xen:include template="ad_thread_view_between_threads" />
</xen:if>
<xen:include template="thread_list_item" />
</xen:foreach>
</xen:hook>