I have the same problem :(
Unfortunately the fullimplementationof theWebformsRootDesigner is only apart of visual studio and is not accessible the rest of us. We do however have access to the abstract WebformsRootDesigner. Unfortuanaly the GetView function returns null. :( So any one wanting to use this rootDesigner is still stuck with writing their own getView.
We have decided to translate windowsform user controls to aspx syntax because trying to make WebFormsRootDesinger fully functional was going to betoo daunting of a task.
I believe the team doing SharpDevelop is in the process of making WebFormsRootDesigner fully functional but i don't know where they are at at this point. But it's going to be something I’m waiting for if they ever do get it done. ;)