O mecanismo de filtro já está em 24.4. Graças a glucas. link
(defvar kill-ring-entry-length 3)
(defun my/replace-blank-kill (args)
(let ((string (car args))
(replace (cdr args))
(last (car-safe kill-ring)))
(when (or (and last (string-blank-p last))
(< (length last) kill-ring-entry-length))
(setq replace t))
(list string replace)))
(advice-add 'kill-new :filter-args #'my/replace-blank-kill)