Overriding pkgs for minimal ISO

I’ve been working on and off on a PR that lets us use uutils-coreutils in place of GNU coreutils and instead of building random packages, it occurred to me that I can try building the ISO and overriding pkgs to become pkgsUutils. I tried modifying various files in nixos/ and pkgs/top-level to no success, how can I achieve this?

building an iso can be done by defining a NixOS configuration that imports one of the modules from nixos/modules/installer/cd-dvd (which one is a matter of taste), and then building that configuration’s attribute config.system.build.isoImage.

That’s the high level anyway, haven’t done it in a while so I might be forgetting some details.