Certainly there's no way to decompose say ac-fn into its own module

It is possible (though somewhat awkward) to create your own forwarding mechanism.

Module A:

  (define ac-fn #f)

  (define (set-ac-fn! fn)
    (set! ac-fn fn))

  (define (ac s env)
    ...
    ((eq? (xcar s) 'fn) (ac-fn (cadr s) (cddr s) env))

  (set-ac-fn!
   (lambda (args body env)
     (if (ac-complex-args? args)
     ...