Hi, Gregorius, and good morning, Jason!
I know New Athena Unicode is a free font which has pre-made symbols with both macron and accents
& breathing marks, though there is no other free font with such symbols, as far as I know, and there's
no key combination to insert them other than the usual hex value alt+[x], where x is a 4 digit number
or a combo of numbers and letters located at the bottom of the symbol window assigned to each symbol
as you click it. Of course, you could change its shortcut to something easier to remember at any time.
You should keep in mind, however, that this font looks awful.
If only we had the complete version of GFS porson used in Smyth and other grammar books.