I have run some experiments by modifying the WPF sample. The second alternative you suggest is basically working. The source code for the PlatformWebView provides the important clue about how the WPF Window and Grid parents can be replaced with a user provided Control (I used a Border in my test). I now have Lock appearing inside a Border inside the XAML in a layout position and size of my choosing.
The way the sample code inspects the Uri in the browser’s LoadCompleted event to know when to return Success is rather obtuse.
I will later package what I have learned into a tidy control or sample and reply here with a download Uri. I know where are other people who need a login UI that integrates more smoothly into their Windows desktop programs.
Thanks for your concise advice that finally led to a way forward on this.
Greg