ありがと! こうしてみたよ。

(define-minor-mode point-indication-mode
"Toggle Point Indication mode.
With arg, turn Point Indication mode on iff arg is positive.
When Point Indication mode is enabled, the point appears
in the mode line."
:init-value nil :global nil :group 'mode-line)

(unless (member '(point-indication-mode
(:eval (concat "P" (number-to-string (point)))))
mode-line-position)
(setq mode-line-position
(add-to-list 'mode-line-position
'(point-indication-mode
(:eval (concat "P" (number-to-string (point)))))
t)))