There is no discussion whatsoever as there is no getting around that milk is produced by a cow is meant for another cow (another species) "period". That you choose to "ignore" that however you "rationalize it" is a whole different story.
A juice fast is just that >juice< and there is no place for fat of any kind in a juice fast "period" and that involves a whole set of different digestive juices and functions that will work at odds with the main purpose of a fast which is to cleanse the system and give the digestive system a break so the energy used for digestion is used to heal. People can "choose" to do whatever or add whatever. It's a matter of what is the best and most efficient way to detox and adding fat to a juice fast is not, "period".