Microsoft Patterns & Practices did not add any information on Extending Unity in the Unity Documentation and on the web. Apparently this was done so that Unity could be released earlier rather than later. In the meantime, you can catch the following webcast on Thursday, July 24, 2008 while the Unity Documentation is being updated:
Extending the Unity Dependency Injection Container
Unity is a lightweight, extensible dependency injection container from Microsoft patterns & practices. This webcast will discuss the underlying Unity architecture and its extension model. We’ll walk through several extensions starting with simple customizations (including custom lifetime managers and custom resolution strategies) to adding completely new features to the container.
You can register for the Unity Webcast here.
If you haven't done so already make sure you check out the previous two webcasts that are a part of this series:
I haven't discussed Unity Extensibility, but if you are looking for information on Unity, you can check out a number of my screencasts:
Hope this helps.
David Hayden