My Return key is getting a bit flaky, so I've just worked out how to reuse my "AltGr" key as a backup in case of emergency. Noting it down for my own reference.
In Linux you can configure anything, including custom keyboard layouts, but it can be tricky to find the advice online that actually matches your system. (Standard Ubuntu has some helper apps that a LXDE desktop doesn't, etc etc.) I think this method will work with any Debian-based system, and I took it from Debian's multimedia-keys guide.
Basically, follow the instructions in that wiki page:
That's enough to enable it for this session. Then you just have to add that last command as one of your autostart programs so it'll be available in future sessions. I did it by running "gnome-session-properties" tool, but also belt-and-braces in my .zshrc because of some other config I have in there.