This is really clever and I like how this can shave a bunch of time off. I thought about doing this for the initrd contained in the top level store path, too.

One problem this presents for me is I actually do need the kernel and initrd in my resulting images: I'm using this to install NixOS to machines, and I want the kernel I install to be pre-loaded in the netbooted image at bootup. Similar story for the initrd.

I also think this would break uses of the store path DB registration by making closures incomplete.

I wonder if there is a way to support transformations like this a bit more arbitrarily without making a lot of special cases?

yeah, that should probably be in the nix expression rather than the server. much more flexible.

also the rdinit argument is probably incorrect (it directly jumps to stage2). that would also suggests the import <nixpkgs/nixos> is not completely appropriate either.