This is worth documenting, if only for the fact that I've never been game to touch the IIS metabase before.

The Communicator Web Access planning document talks about Metabase settings, but not how to change them. Here's the basics -- on the server with the Web Communicator installed:

1. Start a command prompt
2. Type WBEMTEST
3. Click Connect -> Type root\default\winrtc_repository -> Connect
4. Click Open Class... -> Type MSFT_CWASiteSetting -> OK.
5. To edit a field (eg. DefaultSearchField), scroll down to it and click Edit Property.
6. If the value is NULL, click (*) Not NULL to enable editing and type the desired property value in this field.
7. Click Save Property to save (I suspect and probably restart IIS for changes to take effect, but not sure about that bit).

(The vital WMI root information was found here, by the way.)