Seems you would need to first override ocaml and then override the ocaml used in mkOcamlPackages. [1]

  let o = ocaml.override { flambdaSupport = true; }; in
  let oP = ocaml-ng.mkOcamlPackages o (self: super: {}); in
  oP.apron

[1] https://discourse.nixos.org/t/install-ocaml-flambda-compiler...