[plt-scheme] MzLib's unitsig.ss

Is it possible to use macros in a unit/sig?

i.e.:

(define-signature sig^ (my-macro))

(define unit@
  (unit/sig sig^
    (import)

    (define-syntax my-macro
      (syntax-rules ()
        ((_ . ?pattern)
         (foo '?pattern))))))

(which doesn't work -- the example was only to show what I wanted to do)

Or do I have to do something special with the signature?

Or can I simply not do this?