For clarification, I want to do something like this. Note the cascade="all" in the main class, but not in the FooTemplate subclass.
Code:
<class name="FOO">
<component name="subpiece">
<set name="bars" cascade="all">
<key column="FOO_ID"/>
<many-to-many column="BAR_ID" class="Bar"/>
</set>
</component>
<!-- For the template we don't want to cascade deletes -->
<subclass name="FooTemplate">
<component name="subpiece">
<set name="bars">
<key column="FOO_ID"/>
<many-to-many column="BAR_ID" class="Bar"/>
</set>
</component>
</subclass>
</class>