My understanding is that regional advantages only provide a build discount, they do not provide any additional building priorities.

What you are looking for is in collumn 16 in the Crusader_Unit_Prod text file. This allows you to add building bonuses which tie back to the regional advantage (collumn 13). Collumn 16 ties back to collumn 13, but only one region by the way and they do not look at which factions can actually build the unit or any other type building restrictions.