A Bloom filter is absolutely the right way to do this sort of thing, except that I had as a firm requirement, "something that any programmer of any skill level can use in any environment."

The file is small enough, and the approach straightforward enough, that for instance somebody could write a trivial Wordpress or Joomla or Drupal plugin for it, and they wouldn't have to think about Bloom filters or have ready-made lookup functions.

I placed greater value on adoptability than cleverness.