This doesn't have to upset caching. The Last-Modified: of any given
"virtual document" is simply the latest Last-Modified: of its inputs,
if those are known. So if you know the modification date of the directory
and the modification date of any other files you use as input (and the
modification date of the script itself, if you intend to change it...),
you can give a Last-Modified: header in the response.
Dan