RhinoApp.GetPlugInObject with non-RhCommon importer returns null

Is this COM visibility thing true for Grasshopper plugins too?
I tried to make a docking panel with Eto.Forms.Panel in rhino, point to it from Grasshopper and update controls, like changing a Label text or reading values from a TextBox. I got a “get returned null” error.

I referenced the rhino plugin dll and found it with the PlugIn.Find(). The Panel is a public property of the rhino plugin. Panel is showing. Perhaps I just need to correctly find that instance