> If you’re on something with multiple virtual consoles (like if you’re sitting in front of a conventional PC) then you could switch to one of those after you’ve entered the MD configuration part and modify /tmp/mdadm.conf then. I don’t have that option because I’m on a serial console.

debian-installer starts a screen session these days when installing over a serial console.

You can reach the previous and next consoles with Ctrl-a n and p.