John Cowan wrote:
> In the Catalan case, it happens to be simple: [...]

*If* the reader knows Catalan... But you cannot always assume this,
specially in bilingual dictionaries.

> My Italian textbook, IIRC, uses a dot below to indicate the placement
> of stress in words written without an orthographic accent.

So, it doesn't indicate the open or close pronunciation of "o"'s and "e"'s,
treating Italian as it only had 5 vowels.

> It also uses
> the same diacritic under s and z to indicate the voiced pronunciation.

This is common also in Italian monolingual dictionaries. However, this dot
is not part of the ordinary orthography, so there is no possible doubt as to
whether it should be written or not.