Well there's some burly sailors out there on the high seas, so it's not out of the question that Barret would be able to find an accomodating outfit. But the gun arm? Not really sure how you could obscure that, and I'm guessing Barret was fairly notorious amongst Shinra at the time.

Damn, I'm kinda wishing SE made a shooter starring Barret, not Vincent. I might've cared about it more lol.