Sorry for the late response! I totally missed your reply.
The reason for this is the ShowSemiModal expects a generic version of Dialog for its first parameter, not just Dialog. Second, it actually expects three parameters even though the error specifies it requires two: Dialog[T], RhinoDoc, Window
I got it to work using the following script:
self.Content = Eto.Forms.Label(Text="Woodle")
dlg = MyCustomDialog()
Rhino.UI.EtoExtensions.ShowSemiModal(dlg, Rhino.RhinoDoc.ActiveDoc, Rhino.UI.RhinoEtoApp.MainWindow)
I hope this helps!
BTW, creating a new post on discourse for any further issues would be greatly appreciated as we have tools to make sure they are responded to and resolved.