Is there some code or practice I'm not thinking of or is this just something someone hasn't really thought about? If there is no code to the contrary my choice is never to place an air source that will be sucked back to the return in a room for the purpose of a water closet. Same principal almost when you put a vent into a half bath that is located right off the kitchen and usually has no outside wall. Why the need for a Heating and Air vent there as that room is going to assume the temperature of the area around it which is already heated and cooled and thereby you eliminate the possibly that you will suck distasteful odors from the half bath through the kitchen on their way to the Central Heat and Air return.
Maybe it's much ado about nothing but I'm just curious about it.