I imagine for the mag'har the priest specs will be in the reverse social positions to what they usually are on Azeroth- shadow priests from the Shadowmoon Clan as the public norm while holy priests practice their faith in secret. Of course once they arrive on Azeroth things might be different and I can imagine some light-worshipping Mag'har "coming out" and making common ground with light-worshipping Horde members like the Blood Elves, though probably at the expense of being shunned by their own.