So there are some who would allow a Muslim to be a CO in a conflict against other Muslims. And to do so whilst serving in the UK Armed Forces and subject to UK Military Law. And UK Military Law is based on UK Civil Law and gives no one Religious Faction special status over another.
Would those who find favour with this think it is therefore alright for Christians and Jews to seek CO status in conflicts against other Christians and Jews?
Try it and see how far you get.