I would like to spider a site and substitute a portion of 
the filenames found while spidering in the download request.

For example: the site may have references to 
tn_filename.jpg. I would like to have HTTrack remove 
the "tn_" and fetch the resulting file (filename.jpg) in 
addition to or INSTEAD of the tn_ file.

This would allow a site that presents only thumbnail images 
to be mirrored, and have the mirror include the full size 
images behind the thumbnails.