[XProc 3 port@pipeline] Usage of such a shortcut

Hello !

I can use, on a p:with-input, a shortcut to design a pipe's port :

<p:with-input name="..." pipe="port@named-pipeline"/>

But, am I allowed to use this syntax on any XPath expression ? For 
example, may I write this :

<p:declare-step name="named-pipe">
   <p:input port="source" primary="true"/>
   <p:output port="result" primary="true"/>

   <p:xslt message="*Transforming {base-uri(source@named-pipe)}*">
     <p:with-input port="stylesheet" href="../xsl/identity.xsl"/>
   </p:xslt>
</p:declare-step>

Or, if I can't, is there an XSPec XPath extension function that returns 
the content of port@named-pipe ?

Thanks in advance,
Christophe