Desirous of Change,
Incidentally, you will not find the term "Separation of church and state" in the First Ammendment.....or anywhere in the Constitution or the Ammendments. It's a made up term to make people think there is a Constitutional arguement against the "State" getting involved in any way, shape or form with religion. In actuality, the Constition of the U.S. prohibits the establishment of a state religion and allows Americans to practice whatever religion they choose. Anything more than that has been adjudicated and interpreted into the Constitution even though it's not really there.
That being said, I don't believe you will ever see the U.S. government get involved in the disfellowshipping/ shunning debate. There is a concept of "Freedom of association" that allows people to associate with those they choose to associate with and to not associate with those they don't want to associate with.
The only debate I can see them interjecting themselves into is blood transfusions if it involves a minor (which they have done for years), or an adult with diminished capacities. They will not force an able bodied person of sound mind to accept a blood transfusion if they don't wish to.