To (perhaps) answer my own question: Looks like PureOS has
systemd, and so the way to tell the system to boot in non-graphical mode is^1:
sudo systemctl set-default multi-user.target
(as opposed to the current default of
Tested: this does, indeed work. Laptop boots to text-only login prompt.
systemctl command above just sets a symlink in
/lib/systemd/system/, and, if you look in that directory, there are all sorts of interesting
.target files in there, including symlinks for the various runlevels. I don’t know enough about using
systemd to grok the details, but I reckon you could do other interesting things with this.
Any alternatives, or considerations I’m missing, do let me know.