Some sort
of background process to run a couple of times a day and update
per-directory equivalents of Whatsnew.txt could work I suppose; any "dir
listing" code will then only need to operate on a small file. Such an
approach "feels" messy though, but might be the only option without a
full db.
Apache has the ability to associate descriptions to filenames for the
directory indexes it can produce (if you don't have an index.html file, and
you allow directory listings to be produced). Look up the "AddDescription"
directive, and such directives can be added to a .htaccess file in each
directory.
Is it customisable as to where and how it pulls the description information
of of a file for inclusion, though?