From http://www.w3.org/XML/XProc/2006/05/04-minutes.html

HT: Consider that I want to do something like this: take the output of Richards write input to temp file component and mediate between that and a component that will substitute the value into a document.
... I have a general purpose component that has two parameters, an XPath and a string. Wherever the XPath matches, it replaces what it finds with the string.
... The obvious thing to do is set the value of attributes or the text content of elements.
... Now I can get very close to doing the XInclude example if only I can get the result of Richards temporary component into a variable so that I can get it into a parameter.
... That's appealing to some fairly natural concepts of pipeline variables and ways of using pipeline variables to supply component parameters