Suggestions for redacting elements?

I would like to be able to mark certain elements for redaction and  
then have those elements specially handled during the transformation.

For example, given the following...

         <mediaobject>
             <imageobject>
                 <imagedata scalefit="1" fileref="./figures/ 
Shield.gif" scale="100"/>
             </imageobject>
         </mediaobject>

...or...

	<para>Blah, blah, blah</para>

I would like to mark the mediaobject and paragraph for optional  
redaction, such that if a variable (redact=1) is passed to the  
transformation, the elements marked for redaction are replaced with  
text like "REDACTED".

It looks like this might be similar to paragraph numbering as  
documented in Bob's book, but wondered if anyone has already done this  
and might want to post their solution.

-David