Johans,
You're absolutely correct in not wanting to touch the UserBox Control directly in YAF.Controls. We customize UserBox and a few other YAF Controls in Sueetie, as well as do some other customization so we use a separate Sueetie.Forums class which references YAF.Classes.Config, YAF.Classes.Data, YAF.Core, YAF.Types and YAF.Utils. Then we can use code excerpts from various YAF projects as needed.
You can download the Sueetie Open Source Solution called Atomo if you want to see how we're doing it. Attached is a screenshot of our Sueetie.Forums class project in Visual Studio.
Good luck,
Dave
http://sueetie.com
daveburke attached the following image(s):