I think you always need to specify the path starting with the owning entity in a widget.
<property key="child" type="entity" required="true"> </property> <property key="association" type="entity" required="true" isPath="yes" pathType="reference" entityProperty="child"> </property>
This means you need to separately specify the entity on the <many> end. Then set the association from that entity back to the other entity.
It's not perfect as the modeler will not validate that the selected association really targets the other entity, but it works.