That's an interesting recommendation that sounds valid. I'm unsure of the user experience - in particular whether a user is likely to be confused by being thrown back to the system unlock. Any notes on non-techies dealing with this in your experience?

You are right, but this is easy to solve. The less nice solution is to show a little explanation before locking the device, the better one depends on OSes. They should simply provide a way for devs to trigger the lock screen for this specific purpouse and let devs specify a message, color scheme, theme, logo, whatever that needs to feel the lock mechanism more integrated in the app.