iterating through a hierarchy with a filter

[Alexy Khrabrov <deliverable@xxxxxxxxx>]

Greetings -- I have a series of XML files scattered around a  
hierarchy.  I also have a filter script which reads stdin and writes  
stdout in a standard Unix way.

I need to create a mirror hierarchy where files are results of  
applying the filter to the originals, replacing the original  
extension, say .xml, with the result extension, say .txt.  Except for  
the extension change, the hierarchy should be preserved.

Ideally, my script would not know anything about this whole process,  
so I can take any filter and use it for my transforms.  The hierarchy  
for the transformed tree must be created anew so I can easily throw it  
away (i.e. we do not output the results next to the originals).  The  
original and result extensions should be provided as a parameter to  
the process.

Which zshfoo can I use for it above and beyond find with exec helper  
script?

Cheers,
Alexy