The following code compiles correctly:

----- a.ml -----
open Batteries
module Ord = struct type t=string let compare=compare end

module type S = sig
  include module type of Map.Make(Ord)
  (* include module type of Map.Make(Ord).Labels *)
end
-----

An easy workaround is to name the functor's result:

----- a.ml -----
open Batteries
module Ord = struct type t=string let compare=compare end

module M = Map.Make(Ord)

module type S = sig
  include module type of M
  include module type of M.Labels
end
-----

The above compiles correctly, but is this the best/only solution?