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Try this:

(define-key mode-line-buffer-identification-keymap [mode-line mouse-3] 'list-buffers)

Or if you really want to show all buffers:

(define-key mode-line-buffer-identification-keymap
            [mode-line mouse-3]
            (lambda () (interactive) (list-buffers nil (buffer-list))))

However, I agree with those who commented that this is not so useful.