As I wrote before, by default when you setup assigned access the UI only shows app which have the aboveLockScreen extension registered. I talked to the PM who owns this feature to ask why and the reason is because we changed the behavior of how assigned access works (lock the screen and launch the app above the lock screen) we found out a lot of existing apps crash when running on the lock screen, so we changed the UI to only show apps which are designed to run above the lock screen.
edit; this now just works for any application in the latest version of Windows 10. So the manifest is not needed anymore. If you are trying to setup assigned access in Windows 10, you go to settings, accounts, Other Users and there is a link to ‘set up assigned access’. First you need to create a local account you can use for assigned access. You can do that at this ‘Other users’ dialog as well.