Marcin Borkowski <mbork@mbork.pl> writes:
> 2. We modify Emacs itself to somehow highlight the ZWS.  There is (kind
> of) a precedent – a no-breaking space is already fontified with
> =nobreak-space= face.  At the very least, make whitespace-mode somehow
> show ZWSs (which it doesn't now, and I'd probably say it's a bug).
>
> I know that my point 2. is a bit controversial, since it could lead to
> alignment issues where a ZWS is displayed as something with a positive
> width. OTOH, even now changing the face of a ZWS leads to a narrow
> (1-pixel wide) line of a different color.  Is there a way to make it
> a bit stronger?

We can try to create an accent. Try the following:
1. Open new empty org buffer
2. Disable font-lock-mode
3. M-: (insert (compose-string "a​" nil nil (list ?a '(bl . tl) ?␣)))

The result will look like on the attached image.

Best,
Ihor