I'm able to define attribute-ref with the / separator but it is not yet supported.

<cms:attribute-ref name="label"/>
<cms:attribute-ref name="composite_or_repeater_or_content/label"/>

CMS is starting normally without errors but when I display this kind of view we have the label of the content twice.

Note that if the composed attribute-ref is defined under another attribute-ref, the last attribute is the "holder" of the sub-attribute.

Also, we shouldn't be able to define the same attribute twice in a view, but with this method, it is possible.