In an extremely open-minded setup, where any class is prone to change, not only their structure, but also their metaclass, it would seem that a local declaration of the kind (DECLARE (MY-CLASS FOO)) would be totally useless.
Slava Pestov did a few really nice things with Factor. In particular, he had a mechanism that would dynamically trigger recompilation when "open coded/inlined" details changed (e.g. macro redefintion). This declaration would be a good point to store such a hook in CL...
http://factorcode.org/
http://docs.factorcode.org/content/article-vocabs.refresh.html
http://docs.factorcode.org/content/article-tuple-redefinition.html