Preprocess ?

Hello list,

I would like to make a kind of "preprocess" to my docbook's files but I don't know How to do that. Let me explain.

Inside my sources file there is a lot of 
<imageobject conformance="custom_size">
	<imagedata fileref="picture.tiff"/>
</imageobject>
<imageobject conformance="USLetters">
	<imagedata fileref="picture.png"/>
</imageobject>

The value of profile.conformance is defined inside my custom stylesheet.

The problem is how can I generate a docbook file (not a fop one) with only the right imageobject ?

Thanks in advance,
Jacques