While an interesting idea, this isn’t possible because the actively displayed contact list is a private setting of the Contacts app. Third party apps like Locale cannot modify settings within the contacts app for security reasons.

After discussing this with Google, it is clear that third party apps will not be given access to modify the home screen.

Adding and removing widgets on the home screen is only something that the Android OS itself is allowed to do.

This was considered very early in the design of Locale, although was rejected because of the unnecessary complexity it entails.

A setting in Locale is a simple action that can be anything. Like sending an SMS message, posting a Tweet, or changing the ringer volume. Reverting to previous settings is not always possible. Consider the SMS setting: how do you unsend an SMS message?

If only certain settings “revert,” that makes it impossible for users to develop a mental model of a setting, because all settings no longer follow a single, consistent model. With Defaults, Locale always behaves in a predictable fashion.