The Greek Extended block includes precomposed characters for vowels with all known combinations of accents, breathing and iota subscript. It also includes precomposed characters for the vowels alpha, iota and upsilon with macron. (Those three vowels are ambiguously short or long hence the need to mark length in some contexts).
However, there is no precomposition of vowels with accents and/or breathing PLUS macron. (Vowels with iota subscript are always long so don't need a macron to indicate length). This isn't normally an issue in running polytonic Greek text where vowel length is rarely shown but is does occur in lexicons, grammars, etc. I'm wondering what potential objections / problems I should be aware of before trying to put together a proposal for these extra precomposed characters to be included. I wrote a blog post about this issue more broadly (not all of which has to do with Unicode) but which still might be of interest: http://jktauber.com/2016/01/28/polytonic-greek-unicode-is-still-not-perfect/ James