Preferred way to add commands to a foreign keymap?

[Sean McAfee]

I want to add the following command to Dired mode:

(defun mac-open-files (arg)
  (interactive "P")
  (dired-do-shell-command "/usr/bin/open" arg (dired-get-marked-files)))

I can think of two ways to do it:

1.  Twiddle dired-mode's keymap directly:

    (define-key dired-mode-map [(super o)] 'mac-open-files)

2.  Use local-set-key in a hook:

    (add-hook 'dired-mode-hook 
              (lambda () (local-set-key [(super o)] 'mac-open-files)

#1 seems like the (very, very slightly) more efficient appoach, but I
worry that I'm potentially trampling on a dired-mode implementation
detail.

Is there any practical reason to prefer one approach over the other?  Or
is there another way?