Thanks for the follow-up.
To summarize the email thread, it sounds like GCC is used directly by cabal configure, and GCC is patched in nixpkgs to looks at environment variables like NIX_LDFLAGS. nix-shell (or is it stdenv.mkDerivation?) automatically populates NIX_LDFLAGS based on buildInputs. This chain of events causes cabal build to work.
However, in the case of cabal repl, GCC is not used, and GHC is directly responsible for loading and linking libraries. GHC doesn’t know anything about the NIX_LDFLAGS environment variables, so it is not able to find zlib.
The solution you’re proposing is to just modify LD_LIBRARY_PATH so that GHC can find zlib. Given the above info, it makes sense that this would work.
I guess there is also an argument to be made that we should wrap GHC in nixpkgs to make it aware of things like NIX_LDFLAGS. However, in the email thread linked above, peti makes the argument that he would rather have users using ghcWithPackages than raw cabal + GHC.