yes you can use OpenCATS. There's a commitment that any public-facing web page that opencats generates will include the required 'powered by cats' footer.
However - if you use a module (Drupal, Wordpress, etc) to poll the MySQL database directly, then I wouldn't see the need.
Also, where we're reworking the code so that it's no longer the original codebase, we'll flag that the new code is not CPL, but GPL.
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