How are you doing that? In that mode mine (with FV2 on) goes straight to Safari and can't do anything else that I see.
I click on the Apple menu, and I have the option to change the Startup Disk. Given my view (and maybe yours 🙂 of how and why this mode exists, I find the existence of that menu item inexplicable.
In the meantime, I have turned off the Guest/Safari user as I would prefer to not be able to locate my missing Mac rather than make it easier for someone to bang away at my FileVault drive by booting another OS.
A.