Works great on 8.20 @curtisw, thank you!
Easy as this now:
self.BackgroundColor = Eto.Drawing.Colors.Transparent
@clement thank you for all your work on this version below!
# if hasattr(self.ControlObject, "AllowsTransparency"):
# self.ControlObject.AllowsTransparency = True
# # Add Transparent Style To The Form Background Panel
# self.Styles.Add[Eto.Forms.Panel]("transparent", UI.TransparentFormStyler)
# self.Style = "transparent"