Pro (Adobe-Japan1-4) and better Japanese OpenType/CFF fonts included glyphs for upper and lowercase macroned vowels at CIDs 9361 through 9370, and they're encoded at U+0100/U+0101 (Āā), U+0112/U+0113 (Ēē), U+012A/U+012B (Īī), U+014C/U+014D (Ōō), and U+016A/U+016B (Ūū). You don't need any special OpenType feature to access these glyphs. You simply need to use the appropriate Unicode code points.
I would like to add aeiouAEIOU with straight dashes ontop of the characters (like macron marks). These signify long vowel sounds and are used in the Hepbern romanisation of Japanese Language called Romaji.
Looking at a Japanese "Pro" Font:
Unicodes seem to match glyphs for tilde diacritic characters
cid reference codes do not seem to work?
Is there any way to make Romaji work in a font without doing a special function OT feature?