>> But that's not what I meant.  I meant that if we want to base this on
>> text properties, we should do this via hi-lock or similar, not in the
>> display engine which treats all characters the same.
>
> Or markchars.el, or uni-confusables.el.  Like these packages maybe better
> to create another package e.g. nobreak.el, based on font-lock-mode?

Now I extended markchars.el to highlight exactly the same characters
as highlighted by nobreak-char-display, and additionally highlight them
only in files names in Dired.  This is configurable with such hook:

  (add-hook 'dired-mode-hook
            (lambda ()
              (setq-local nobreak-char-display nil)
              (setq-local markchars-what '(markchars-nobreak-space
                                           markchars-nobreak-hyphen))
              (markchars-mode 1)))