advice にしてみた

(advice-add 'mozc-preedit-update :around
(lambda (old-func &rest r)
(let ((buffer-read-only (and buffer-read-only
(not inhibit-read-only))))
(apply old-func r)
)))
(advice-add 'mozc-im-input-method :around
(lambda (old-func &rest r)
(let ((buffer-read-only (and buffer-read-only
(not inhibit-read-only))))
(apply old-func r))))