Here in VA, the attic is unbelievably hot in summer, and you just can't go up there for more than a couple minutes. God forbid if I had to replace the water heater in summer -- I guess I should be thankful I did it in February. But that tells me that it's a stupid place for the HVAC ducts to be in summer when you're running cool air through them.
It would be the opposite problem in winter, with the cold temperatures working against the heat flowing through the ducts. I measured attic temperatures during the winter last year, and they dropped to within a couple degrees of ambient temperature. I was trying to figure if we could install a humidifier on the furnace. But since the overnight temperature up there would routinely drop into the teens, and rarely single digits, that was a no-go. Based on that, I think even having water lines up there is marginal.