Well now you can find out relatively quickly! The Motion Picture Scripts Database offers a searchable database of six area screenplay libraries. In addition to the Herrick and WGF, it also references the American Film Institute's (AFI) Louis B. Mayer Library, the Frances Howard Goldwyn Hollywood Regional Library (of the LA Public Library), the Arts Library Special Collection at UCLA's Charles E. Young Research Library, and USC's Cinema-Television Library (Doheny branch).
Some of these libraries have restrictions on access, but at least you can find out if they have what you're looking for. And as they say, knowing is half the battle. Then get on the horn or use some connections if need be to get in there.
Tags: screenplays, Margaret+Herrick+Library, Writers+Guild+Foundation