Some answers were provided in this question:
breve under letter is not in italics when using \textit
It is clear then, that one can make a custom "breve below" macro. However, is the best way, as of today? That is, are there packages that provide more versatile definitions?
fontspecpackage with XeLaTeX or LuaLaTeX. – stone-zeng Jan 04 '19 at 01:35