This reminds me of several conversations (read: arguments) I've had with D&D gamers in the past regarding paladins. I always maintained that a paladin is a holy warrior granted some magical abilities by his/her god; however, I always insist that the paladin must be the alignment of his/her deity. Meaning that a paladin can be of any alignment as opposed to only being Lawful Good.
As for the original poster's question, I thought it was stated (but it might only be implied) in the Your Story book that you have to be part or wholly mortal in order to wield a Sword of the Cross. I think it has something to do with the White God having purview over mortals and the mortal realm.
As a counter argument though, if there were such things as 'holy relics/items' for RCVs, then they might act similarly against mortals like the SotC work against vamps.
