I have posted a series of blogs on extensibility support in Coded UI Test.
These should get anyone looking for writing extension for Coded UI Test, get started. Use the Coded UI Test forum for questions\more info.
P.S. A related post: 7. Configure search properties used by recorder\code generation
8. Writing extension for WPF custom control
9. Extending supported properties set of a control
10. Writing your own client using UITest Object Model
More posts by Mathew Aniyan on this topic -
11. Custom WPF Control’s custom properties
12. Enabling UI Testing for Third party WinForms custom controls – 1
13. Enabling UI Testing for Third party WinForms custom controls – 2