Finally got emacs-wiki highlighting just the way I want it.
Apparently, it bypasses the normal font-lock thing. Changes: I made
the example tag remove all the properties from the bounded region.
Here’s the relevant snippet from my ../emacs/emacs-wiki-config.el:

(defun sacha/htmlfontify-insert-region (buffer begin end)
  "Insert into BUFFER the htmlified text between BEGIN and END."
  (save-excursion
    (let* ((hfy-optimisations (cons 'skip-refontification hfy-optimisations))
	   (input-text (buffer-substring begin end))
	   (temp-file (make-temp-file "html-input"))
	   output-buffer)
      (with-temp-buffer
	(insert input-text)
	(setq buffer-file-name temp-file)
	(save-excursion (setq output-buffer (htmlfontify-buffer nil nil)))
	(set-buffer-modified-p nil))
      (unwind-protect
	  (let (b e yanked-output)
	    (set-buffer output-buffer)
	    (goto-char (point-min))
	    (search-forward "
\n")
	    (setq b (line-beginning-position))
	    (goto-char (point-max))
	    (search-backward "
")

	    (forward-line -1)

	    (setq e (line-beginning-position))

	    (setq yanked-output (buffer-substring-no-properties b e))

	    (set-buffer buffer)

	    (insert yanked-output))

	(set-buffer output-buffer)

	(set-buffer-modified-p nil)

	(delete-file temp-file)

	(kill-buffer output-buffer)))))
(defun sacha/emacs-wiki-example-tag (beg end attrs highlight-p)

  "Mark up text as an example with optional font-locking."

  (if highlight-p

      (progn

        (remove-text-properties

         beg end '(face nil font-lock-multiline nil

                        invisible nil intangible nil display nil

                        mouse-face nil keymap nil help-echo nil))

        (goto-char end))

    ;; I don't know what would happen if you don't have

    ;; htmlfontify. I guess if you are installing this you

    ;; should have it...

    (let ((end-marker (set-marker (make-marker) (1+ end))))

      (save-restriction

	(narrow-to-region beg end)

	(let* ((mode (cdr (assoc "mode" attrs)))

	       (start (progn (forward-line) (point)))

	       (stop (progn (goto-char end) (beginning-of-line) (point)))

	       (text (buffer-substring-no-properties start stop))

	       (buffer (current-buffer)))

	  (delete-region beg end)

	  (with-temp-buffer

	    (insert text)

	    (when (and mode (and (stringp mode) (functionp (intern mode))))

	      (funcall (intern mode))

	      (font-lock-fontify-buffer))

	    (sacha/htmlfontify-insert-region buffer (point-min) (point-max)))

	  (goto-char (point-min))

	  (insert "
\n")
	  (goto-char (point-max))
	  (insert "
\n")

	  (add-text-properties (point-min) (point-max)

			       '(rear-nonsticky (read-only) read-only t))))

      (goto-char end-marker))))
(add-hook 'emacs-wiki-mode-hook 'emacs-wiki-highlight-buffer t)