My latin dictionary says "alio" is an adverb (not a declension of alius, which is an adjective) which means "to someone/someplace else". I think it was a relatively common latin word.
I just checked my Latin dictionary, and while you're right, Alio does appear in there, it seems to just mean "to another place." I think Obelics in the thread you posted a link to said it best:

According to my old dictionary, the adverb "Alio" exist, but the main meaning is "elsewhere"
Now, always according to it, this adverb has an "extended" II(second) meaning of "elsewhere"="to a different people", but just as a traslated/extended meaning.
So if the english sentence sound good as "Everyone is barbarous elsewhere" than is ok, but i dont think so.
To better undertand if i say to a man: "ehi! I dont want to read your curriculum"
then the guy could answer "then i will send it elsewhere" in the same meaning of "I will send it to some other guy" in this case i could use Alio as an adverb
But i think "Everyone is barbarous elsewhere" or “Everyone is a barbarian to other places” in the sense of "other people" sounds strange to me.
Also even if we want to accept it as a poetical licence, than it's a bit strange (for me) to see an adverb at the end of the sentence.

Here's a quote from Livio:
quo alio nisi ad nos socios

means: to whom if not to us allies? litterally==> to what "other where" if not to us allies?
http://la.wikisource.org/wiki/Ab_Urb...ta/liber_XXXIX

that is what i got from my old lyceum vocabulary, and according to my knowledge that i repeat is very limited (highschool), so as usually dont take it sure.
I think it's generally agreed that "Quisque Est Barbarus Alio" is supposed to mean "Everyone is a Barbarian to Someone Else," not "Everyone is a Barbarian Elsewhere." Therefore, it should be changed to Alii. -M