Surely the intention is for <INVISIBLE LETTER, combining acute> to be equivalent (although it cannot be canonically equivalent) to spacing acute, U+00B4? But then would this kind of ligature mechanism with ZWNJ and U+00B4 be appropriate? I would think not.
<INVISIBLE LETTER,combining acute> will not be canonically equivalent effectively, depite it should produce and behave like the spacing acute.
As ZWJ is intended to indicate that there's effectively a ligature opportunity between two grapheme clusters, I don't see why one would not support <a,ZWJ,SPACING ACUTE> to kern the spacing acute on the right side of a. It won't create an accent *centered* above the letter, but it now allows the accent to move within the spacing area of the preceding letter.
I accept the fact that this is just a ligature opportunity for renderers, with no different semantics than in absence of the joiner. But I wonder if the digraph with the centered accent above is not simply that: the accent is a notation that does not change the semantics of the surrounding two vowels, with no orthographic consideration.
In that case, this is really a rendering feature, and using ZWJ could be appropriate here, notably because <IL,combining acute> will remain canonically distinct from U+00B4, which also has the wrong character properties (not a letter, this is a symbol and a word-breaker by itself...). Most uses of isolated diacritics however are mainly symbolic rather than orthographic. The IL however changes this, and becomes appropriate within the middle of words.
