> Testing shows that the `isearch-buffer-local' change works fine,
> with only one deficiency: as (info "(elisp) Searching Keymaps") says
> get-char-property takes precedence over emulation-mode-map-alists.
>
> This means that typing isearch characters when point is e.g.
> on a Gnus attachment line, typing a character like 'o' bound
> to 'gnus-mime-save-part' will exit isearch and run this command.
>
> I see no way to workaround this limitation.

Here is a patch that allows minor mode keys to take precedence over
char-property keys, by adding a special property to mode symbol, e.g.

  (put 'isearch-mode 'overriding-keymap t)

indicated in the decision tree by NEW:

     (or (if overriding-terminal-local-map
             (FIND-IN overriding-terminal-local-map))
         (if overriding-local-map
             (FIND-IN overriding-local-map)
           (or ;; => NEW: modes with overriding-keymap
               (FIND-IN (get-char-property (point) 'keymap))
               (FIND-IN-ANY emulation-mode-map-alists)
               (FIND-IN-ANY minor-mode-overriding-map-alist)
               (FIND-IN-ANY minor-mode-map-alist)
               (if (get-text-property (point) 'local-map)
                   (FIND-IN (get-char-property (point) 'local-map))
                 (FIND-IN (current-local-map)))))
         (FIND-IN (current-global-map)))