The following code, when put into your .gnus.el, will bind the key
a in such a way that hitting it on an attached Darcs patch will
query for a darcs repository to apply this patch to.

::

    (eval-after-load "gnus-art"
      '(define-key gnus-mime-button-map (kbd "a") 'my-gnus-darcs-apply-part))
    
    (defun my-gnus-darcs-apply-part (repo)
      "Apply the MIME part under point to a Darcs repository."
      (interactive "DApply to Darcs repository: ")
      (gnus-article-check-buffer)
      (let ((data (get-text-property (point)
                                     'gnus-data)))
        (when data
          (mm-with-unibyte-buffer
            (mm-insert-part data)
            (my-send-region-to-command (point-min)
                                       (point-max)
                                       "darcs" "apply"
                                       (format "--repodir=%s"
                                               (expand-file-name repo))
                                       "-a")))))
    
    (defun my-send-region-to-command (beg end command &rest args)
      "Call COMMAND with ARGS, and display output in a special buffer."
      (let* ((coding-system-for-write 'binary)
             (buf (with-current-buffer
                      (get-buffer-create "*Shell Command Output*")
                    (setq buffer-read-only nil)
                    (erase-buffer)
                    (current-buffer)))
             (exit-status (apply 'call-process-region
                                 beg end
                                 command
                                 nil buf nil
                                 args)))
        (with-current-buffer buf
          (setq mode-line-process
                (cond ((null exit-status)
                       " - Error")
                      ((stringp exit-status)
                       (format " - Signal [%s]" exit-status))
                      ((not (equal 0 exit-status))
                       (format " - Exit [%d]" exit-status)))))
        (if (with-current-buffer buf (> (point-max)
                                        (point-min)))
            ;; There's some output, display it
            (display-message-or-buffer buf)
          ;; No output; error?
          (cond ((null exit-status)
                 (message "(Command failed with error)"))
                ((equal 0 exit-status)
                 (message "(Command succeeded with no output)"))
                ((stringp exit-status)
                 (message "(Command killed by signal %s)"
                          exit-status))
                (t
                 (message "(Command failed with code %d and no output)"
                          exit-status output))))))