Oh, FFS! How come I never thought of this

module Univ : Univ = struct
  type t = { 
    id : unit ref;
    store : unit -> unit; 
  }
 
  let embed () = 
    let id = ref () in
    let r = ref None in
    let put a =
      let o = Some a in
      { id = id; store = (fun () -> r := o); }
    in
    let get t =
      if id == t.id then (t.store (); let a = !r in r := None; a) else None
    in
    (put, get)
end