[plt-scheme] units and (export (rename ...))

Why doesn't the following work?

   (define-signature sig^
     (operation))

   (define (make-sig op)
     (unit
       (import)
       (export (rename sig^ [op operation]))))

Obviously the workaround is:

   (define (make-sig op)
     (unit
       (import)
       (export sig^)
       (define operation op)))

Is it fundamental to units that you can only export things that are 
explicitly defined inside the units, or is it possible this could be 
relaxed?

Thanks,
Dave