chattr -i /
mkdir -p /gnu/store
chattr +i /
The last command restores the immutable attribute on the root filesystem. I am able to write arbitrary files (e.g. `touch /gnu/store/test`) even with that attribute set, so it isn't inherited by subdirectories (maybe obvious).Now, persistence of the /gnu/store data could be an issue, I'm not sure, but maybe this gets you started?
EDIT: next step is to get Guix installed and then try to update the system, we shall see how it goes