Author here: I wrote this this weekend to automate the copy-recursively-but-with-extra-processing pattern that I wanted to apply to my music collection and website source. For the website, I was using a make hack I describe here:
http://scrivulet.com/fixed/makefilter.html , but it wasn't going to cut it when I wanted to change the .flac extension to mp3. And anyway, I felt like writing something quick.
Hopefully it's useful to someone, as a template if nothing else. If anyone knows a better tool for this, there's a pretty good chance I'll go for it.