It wasn’t apparent to me from the docs (back then?) how to use the custom libpng only with a specific package. Do you have a pointer to the documentation on that please?
https://nixos.org/nixos/nix-pills/override-design-pattern.ht...
https://nixos.org/nixos/nix-pills/nixpkgs-overriding-package...
Those are the documentation pages I found right now...
It's basically two steps: constructing the patched libpng derivation, and then overriding the libpng derivation in the specific program.
The first step would be something like
let myLibpng = pkgs.libpng.overrideAttrs
(old: { patches = old.patches ++ [~/libpng.patch]; });
let myProgram = pkgs.program.override { libpng = myLibpng; };
If you have an "overlay" set up in your system then you could also do this as part of that overlay.