[racket] How to make unit signature macros take prefix into account

I have a signature that includes a macro to clean up the use of one of the
procs in the signature.

Two different units with the same signature are used
via define-values/invoke-unit - the second one uses a prefix in the export
sig-spec to differentiate it from the first.
The macro introduced by the second use does not refer to the other proc
using the prefix.

The concrete example:

;; Signature of a message channel
(define-signature message-channel^
  (
   ;; ...
   msg-chan-subscribe   ; ( (msg -> boolean) first? -> thunk)

   ;; Enqueue message(s) for a channel at normal priority
   <--   ; ( msg .. -> void )

   ;; Enqueue message(s) for a channel at high priority
   <++   ; ( msg .. -> void )

   ;; Macro to create a message handler.
   ;; ...
   (define-syntaxes (==>)
     (syntax-rules (msg)
       [(_ match-body ...) (msg-chan-subscribe
                            (λ(msg)
                              (syntax-parameterize
                               ([the-message (make-rename-transformer
#'msg)])
                               (match msg
                                 match-body ...
                                 [_ #t])))
                            #f)]))))


When (define-values/invoke-unit <some-unit> (import) (export (prefix
diagram message-channel^))) is used it introduces the bindings diagram<-- ,
diagram<++ , diagrammsg-chan-subscribe and diagram==> .

The form

(diagram==>
   .. message handlers ..)

expands to (msg-chan-subscribe ...) rather than (diagrammsg-chan-subscribe
...) - and thus it listens to the first (wrong) message channel.

Is there any way to make macros defined in signatures be aware of the
prefix used with the signature ?
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