polytonic Greek: diacritics above long vowels ᾱ, ῑ, ῡ

[Stephan Stiller]

Hi,

If one wants to indicate vowel length for the length-ambiguous vowels α, 
ι, υ in Ancient Greek, one writes ᾱ, ῑ, ῡ. Is there a reason for why 
there are no diacritic-precomposed characters? I guess it's because 
macron usage is rare in orthographic practice, even though vowel length 
here is not clearly less important than the other phonetic aspects 
indicated by the various diacritics in use in polytonic orthography. 
Thus I am wondering when and how the relevant decisions were made.

ἀγορᾱ́ ("gathering, marketplace") needs a combining diacritic at the end. 
ῑ̓́φιος ("fat, strong") has ῑ̓́ (which should be thought of as ῑ with two 
combining diacritics: U+1FD1 U+0313 U+0301). Multiple combining 
diacritics work for me, but I'd like to check whether there are further 
thoughts on this topic.

Stephan