Aqui está uma função que copia o conteúdo do buffer atual em um novo buffer e executa o modo hexl em:
(defun hexify-buffer-copy()
"Edit current buffer in hexl mode by copying it"
(let ((orig-buffer (current-buffer)))
(switch-to-buffer (create-file-buffer
(buffer-file-name orig-buffer)))
(insert-buffer orig-buffer)
(setq buffer-undo-list nil)
(hexl-mode)))
Executar com M-: (hexify-buffer-copy)