Patching chromium from nixpkgs

Hi! I’d like to patch the source code of nixpkgs’ chromium using an overlay (yes, I really want to build chromium). The core chromium derivation (the result of calling mkChromiumDerivation seems to be wrapped inside another derivation, so simply calling overrideAttrs on the top-level chromium attribute won’t work. I can’t seem to find a way to patch the core derivation using just an overlay. Has anyone of you been successful in doing that (or is it even possible)? Thanks!