Looking at the Google Assistant developer platform docs, and perhaps I missed it. What I would like to do is essentially build a kiosk "app" -- one that is up on the screen by default. E.g. when you go to Best Buy and they have their demo app up on the computers as you shop.
So, on a Google Home Hub device that this app is installed on, it would be the "default" app -- like how photos show up after a period of inactivity.
Is this possible?
No, the platform does not have a way for third-party developers to build an application that runs on the "homescreen" nor is there a way to change the homescreen application.