Greek diacritics
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According to Wikipedia (https://en.wikipedia.org/wiki/Greek_diacritics#Position_in_letters): "Diacritics can be found above capital letters in medieval texts".
Maybe Junicode could add an OpenType feature for Greek diacritics above capital letters. I had a quick look at the samples PDF and it doesn't seem to be there.
In the next version (coming soon), Greek will be a separate font. It will have anchors to enable users to place accents over capital letters if they prefer that to the style currently used.