2. Another invisible diacritics carrier
I also found an acute on diphtongs, placed on the boundary of both letters (au, ei, eu, oe, and ui).
Wouldn't such diacritic be hold by the currently proposed invisible base character (in the Public Review section of the Unicode website), by encoding for example:
a,INVISIBLE LETTER,combining acute,u
If you think there's a grapheme cluster here, I suggest using ZWJ to attach the three default grapheme clusters:
a,ZWJ,INVISIBLE LETTER,combining acute,ZWJ,u
to create a kerning ligature between the two vowels.
The invisible letter in PR-41 is also intended to support the "INV" character found in ISCII for standard Brahmic scripts of India, but with probable interoperability problems.
But I currently do not see indication for its correct usage in the Latin script, except as a way to transform a combining diacritic into a non-combining one in isolation, when the legacy use of SPACE causes interoperability problems such as in XML and HTML or with word-breaking algorithms.
As the intent is to create a spacing diacritic, not using a joining/ligaturing control before and after it would not create the desired effect, as the acute above would be shown on a blank space between 'a' and 'u', as wide as the acute accent itself.
The PR-41 proposal document suggests that the typical use of the Invisible Letter would be to display a isolated spacing diacritic between two spaces (or punctuations), a case where the XML/HTML treatment of whitespace sequences is to collapse them before rendering or interpreting them.
Your request is quite similar to the case of "double diacritics" already encoded in Unicode, except that double diacritics are displayed on the whole display width above the two letters, when your usage would just be to put a standard width diacritic centered on the kerning space between them. For an acute accent, it's unlikely that doubling its width would be very readable, where it could be confused with a macron. May be your centered diacritic should be encoded like the other double diacritics.
