I’m trying to arrange that it is a read only file system, with overlayfs, similar to the ISO.

It does boot, but the duplicate mount is causing problems.

Thanks for the hint; could this explain the oddity I'm seeing?

Essentially, I have a ro store, that I mount at /nix/rostore, and then a writable partition I mount at /. I want to make /nix/store be an overlayfs with /nix/store (on the writable partition) with the rostore as the lower layer. I have something that works, but then the /nix/store directory appears empty if you try to list the files in it. Directly opening the paths is fine.

I can't figure out why I can't get a sane overlayfs mount..

Something like:

fileSystems = {
  "/" = {
    device = "/dev/disk/by-partlabel/nixos-rw";
    fsType = "btrfs";
    options = [ "x-systemd.growfs" ];
    autoResize = true;
    neededForBoot = true;
  };
  "/nix/rostore" = {
    device = "/dev/disk/by-partlabel/nixos";
    fsType = "squashfs";
    neededForBoot = true;
  };
  "/nix/store" = {
    fsType = "overlay";
    device = "overlay";
    overlay = {
      lowerdir = ["/nix/rostore/nix/store"];
      upperdir = "/nix/store";
      workdir = "/nix/ovfs-work";
    };
  };
};