OK, this is what I'm trying to achieve:
Have a separate menu option in GRUB, so that I can select it and boot straight into the command line – that is: without a desktop manager, but leaving the existing menu item (to boot Ubuntu as usual) intact.
I've looked into
/etc/default/grub but there seems nothing to do there. And also
/boot/grub/grub.cfg which is auto-generated, so I don't think it would be a good idea to edit it directly.
How should I go about it? Any idea?
Update: The existing answer (suggesting that this question was a duplicate) was obsolete. For the correct solution, please see the accepted answer below, which works beautifully. (at least for 16.04+ as it seems)