[jcranmer]

Jamo, Emoji (including flag combinators), Arabic, and Indic scripts all combine according on effectively per-character basis. There's not really any existing character that says "display any Unicode grapheme A and grapheme B in the same visual cell with A above B." The proposed additions to Egyptian hieroglyphs would be the first addition of such a generic positioning control character to my knowledge, albeit perhaps limited just to characters in the Egyptian Unicode repertoire.

Research on what to do vis à vis Mayan characters (including perhaps reusing Egyptian control characters for layout) is still ongoing, as is better handling of Egyptian.