I don't think the Supreme Court has ruled on this issue. I would think that if the issue came before the supreme court that they would state categorically that if a city has a religious heritage that it would be perfectly permissible for that city to reflect that heritage in their official seal as it would serve a secular purpose to recognize that heritage.
My guess is that if one of these cases made it to the supreme court that the SCOTUS would rule unanimously that a cross on the seal of a city founded by missionaries from the Catholic Church was constitutionally permissible.
http://www.cwnews.com/news/viewstory.cfm?recnum=498
After this the ACLU went after another suburb called "Bethany". Bethany is a little town within Oklahoma City that was founded by the Nazarene church. Built around a Nazarene college. The symbol of the college, among other things was on the city seal. This had to be removed. The city lost in fed court but decided not to appeal to the Supreme Court because of the cost, and because of Edmond having just gotten kicked.
LA county will not win this in court. Until we take back our courts, we cannot live in freedom.