You need to evaluate whether or not to use non-UEB passage indicators (5 12356 3) to open and (6 5 23456) to close. These code switch indicators are found at the beginning of Section 14: Code Switching in the Rules of UEB, Look up Sections 13.7.2 and 14.3.2 also. These will state that you can omit the non-UEB passage indicators as your Greek is in bold. However, it may be helpful to use the non-UEB passage indicators if your reader is unfamiliar with Greek.
In any case, you will be using the contractions listed on page 186 of World Braille Usage for your acute and grave vowels. Remember, the Greek passage is uncontracted.
There is a NBA Bulletin article that covers breathing marks in Greek titled Transcribing Breathing Marks in Greek: English Context. It was published in the Fall 2016 NBA Bulletin edition, Volume 52, Number 3.
Hope this helps,