how about this rule:
<iso:pattern id="check IDs">
<iso:rule context="TreeView">
<iso:assert test="parent::*/parent::*/Pages/Page/@id = @id">
TreeViews/TreeView/@id = <iso:value-of select="./@id" /> NOT FOUND in Pages
</iso:assert>
</iso:rule>
</iso:pattern>
Hope this helps.