>Trick the user into running a program that does 'alias sudo=evil-sudo' >> ~/.bashrc
That's only going to get you the user's password, not the root password.
That's only going to get you the user's password, not the root password.
An attacker might still bring an evil-su in addition to an evil-sudo, though. And even if you're logging into that root user only in an another tty, it seems like an unnecessary risk to share the password with LUKS.