I don't see why Sha'ir can't work. It doesn't really matter if a character from the Middle East might not fit into the campaign, what matters is the class itself.

We name plenty of things based on their country of origin that are no longer exclusively from that country.

Sha'ir is what the people who came up with the practice named it, and so that is what foreign practitioners of it also call themselves.

Are Druids the world over required to prove their Celtic roots to practice? Are Magi exclusively from the remnants of the Roman empire equivalent? Are Paladins all required to be from a Germanic cultural root?

I don't feel like anyone reasonable would claim so. So why would Sha'ir be any different?