submodule-with-parent-config.nix

let
  sub = { config, lib, parentConfig, ...}: {
      options.foo = lib.mkOption {
          type = lib.types.bool;
          default = parentConfig.bar;
      };
  };

in { config, lib, ...}: {
    options.bar = lib.mkOption { type = lib.types.bool; default = true; };

    options.bla = lib.mkOption {
        type = with lib.types; attrsOf (submodule { imports = [sub]; _module.args.parentConfig = config; });
    };

    config.bla.test = {};
}