One of the ways for extending the WebBrowser 2.0 control is to inherit from WebBrowserSiteBase. According to the documentation, you have to override the method CreateWebBrowserSiteBase and return your specific implementation of WebBrowserSiteBase.

Everything is there to plug-in your implementation of WebBrowserSiteBase, but the problem is that the constructor of WebBrowserSiteBase is marked as internal, and so there is no way to inherit from. I am pretty sure that it was possible with the beta release of .NET 2.0, but for some reason they marked it as internal in the RTM version. This means that implementing some interfaces that are described here are not possible.

I am planning to release a .NET 2005 version of my IStaySharp.WebBrowser project. More info coming soon.