> The way it works is that notmuch unconditionally calls
> notmuch-pick-init when started: in the default case (the stub file)
> this does nothing, whereas if the user replaces the stub by the full
> file this sets up all the keybinding etc.

In case people are interested the relevant code in the real
notmuch-pick-init function is

(defvar notmuch-pick-initialized nil)

(defun notmuch-pick-init()
  (unless notmuch-pick-initialized
    (define-key 'notmuch-search-mode-map "z" 'notmuch-pick)
    (define-key 'notmuch-search-mode-map "Z" 'notmuch-search-pick-current-query)
    (define-key 'notmuch-search-mode-map (kbd "M-RET") 'notmuch-search-pick-thread)
    (define-key 'notmuch-hello-mode-map "z" 'notmuch-hello-pick)
    (define-key 'notmuch-show-mode-map "z" 'notmuch-pick)
    (define-key 'notmuch-show-mode-map "Z" 'notmuch-show-pick-current-query)
    (setq notmuch-pick-initialized t)
    (message "Initialised notmuch-pick")))

Best wishes 

Mark