Onlyfun At present the AOSP system requires that after boot the owner profile is unlocked first. That may change in the future, but that's how it is now.
Also, it might not be good for the login screen to try the same PIN/passphrase to unlock multiple accounts in quick succession, since that might trigger hardware throttling (now or in the future).
Overall, I am skeptical that this idea will be implemented.
Please note that I do not speak for the GrapheneOS project.