Don't quote me on this, but I believe Poser's zip format is actually different than the standard zip, so using 7zip to compress the files might not be such a good idea. Like I said, I could be wrong, but I remember someone mentioning here in the forums the header was a little different, even though it seems to compress and decompress fine, there's a chance the file could end up not working anymore.

Also, Poser already comes with a script for compressing files, and it works recursively on any directory you point it to. Any particular reason why you are not using that script?

As for python, well I can see you are quite comfortable with batch scripting, but I only mention it because it could significantly speed up your work. Take a look at the following example to print the name of all files inside a directory (recursively).

import os
for root,dirs,files in os.walk("C:SomePathWithProperPermissions"):
   print files