.ercrc.el

(defun my-erc-joined-channels ()
  "Return all the channels you're in as a list.  This does not include queries."
  (save-excursion
    ;; need to get out of ERC mode so we can have *all* channels returned
    (set-buffer "*scratch*")
    (mapcar #'(lambda (chanbuf)
                (with-current-buffer chanbuf (erc-default-target)))
            (erc-channel-list erc-process))))

(defun mark-channel (channel)
  (save-excursion
    (set-buffer (get-buffer-create channel))
    (goto-char (point-max))
    (forward-line -1)
    (end-of-line)
    (let ((inhibit-read-only t))
      (insert "\n<<<<<<<<    MARK    >>>>>>>>")
      (center-line))))

(defun mark-all-channels ()
  (interactive)
  (mapcar #'mark-channel (my-erc-joined-channels)))

(define-key erc-mode-map "\C-cm" 'mark-all-channels)