Ephesians 1:6 uses the same word in a different tense. It is the perfect Passive Participle - a verb describing something that was done to Mary not a noun describing what she was.
The Perfect Passive Participle
The perfect passive participle is the 4th principal part of a transitive verb. Perfect passive participles can usually be translated into English with the -ed ending: dux captus = the captured leader ( = the leader having been captured)
Thus, translating this Mary, thou art highly favored is well with in grammatical rules. "Full of Grace" is not. Filled with grace would be.
Ephesians 1:6 is also passive though a different form of the same verb(and the ONLY time it is found in Scripture. Charis is not the immediate root of what Mary was called. Charitow was). Ephesians is translated "made accepted" or grace freely bestowed on us.
In both cases, the subject is the recipient of something that God is giving. It is not a statement of what they are but what has been done to them. Mary was given unmerited favor (Grace) and made accepted to carry the Messiah. She was blessed among women because of this. None of this implies she was without sin. We have been made accepted by the grace freely bestowed on us by God.
Same verb. Two different tenses. Same basic meaning.
"Filled with Grace" would be acceptable, yes. Full and filled are synonymous. The important part is that the "filling" is in the past relative to the moment the angel is speaking.
"highly favoured" is not specific of the past time of the "favor" and it replaces "grace" with theologically insignificant "favor", despite the root being "charis", with primary theological meaning of "grace".
-menos is generally describing the person. For example, "fortomenos", loaded. Likewise, "charitomenos", "graced".