My background is RHEL-derived distributions (mainly Fedora, today), Arch is foreign to me.
mgetty where used by RHEL, you could edit
/etc/inittab and pass the
-p option to
mgetty. This would set the prompt.
Now with the adoption of
/etc/inittab is empty. The tty configuration file is now
/lib/systemd/system/getty\@.service. This file seems to use
agetty instead of
mgetty to manage the tty.
agetty man page doesn't show the
-p option, but you could reinstall
mgetty and use it. Or you could use the
-n (no prompt) option with the
-l (specify a login program) option and write a wrapper to
/bin/login with your own prompt.
Now, I don't know how to set up this on Arch, but you have to find out where it manages it's ttys (
/etc/inittab is the first place you have to look), then see with the tty program how to roll your own custom prompt.