Dynamically setting the text of an added element in a pipeline . . ?

Am I missing something . . . or is it not possible to dynamically set 
the /text()/ of an element in a pipeline (in xproc without having to 
jump out to P:XSTL or P:XQUERY)?

Something to the effect of:

    <p:for-each name="UploadInGroups">
    <p:insert position="last-child" match="/add/doc" name="insertCounter">
    <p:input port="insertion">
    <p:inline>
    <field name="iterationCount">*p:iteration-position()*</field>
    </p:inline>
    </p:input>
    </p:insert>

         . . .

    </p:for-each>

Most grateful,

Christopher