That is an excellent question! The crutch, I think, revolves around "hostile magic". Are you an enemy of the zone? Because you can still enchant your Warded friends. I think most players would say that you are neutral to the zone. The only example we have for dealing with neutral objects, the orb guardians, then tells us to treat them like enemies. Plus, if nobody was an enemy of the zone, what would be the point of it getting the Warded trait?
So, I believe you are correct. The zone containing Septagram can not have any zone enchantments on it. I will admit though, that the rules do not actually cover this. The section on the orb guardians only really applies to them and can't honestly be used in this situation. There is no section covering the allegiances of the zones themselves.