You are most correct that only Southerners have had the good sense to create a 2nd person plural where none existed in the English language. I am a bit of a Southern-o-phile, even though I was born and raised in the intermountain west. I even had the good sense to marry a southern girl (and she had the good sense to move out to the mountains with me.)
But I think there are just too few people who would be willing to use "y'all" in the liturgy, even if it seemed to work.
I would add that even good Southerners wouldn't use it, since they have tended to hold on to traditional liturgical English longer than those in other areas of the country. This may be because they just have good sense, or it may be because they are more naturally comfortable with Early Modern English because of its similarities to Southern speech cadences.
I'm afraid that the Yankees would be just as prejudiced toward the use of Southernese in the liturgy as apparently Latins were toward the use of Germanic languages.
P.S. Wasn't the Wulfilas Bible the product of the Gothic church in their Arian era? I'm not sure that the Western Patriarchate can even take credit for that. As so, historically, we are left with the curious oddity of the Indian languages in North America (but apparently not other non-Indo-European languages in, say, India or the far East.