Either you finish off the attic so that it can't freeze in there anymore, or you don't put water in the attic at all. There's a fair alternative, though- the Aprilaire model 360. It mounts in a wall in the living space of the house, not in the attic where it can freeze up. It needs a connection to a hot water line and a drain. It's technically not a "central" system in that it doesn't distribute through your ductwork, but the airflow associated with having forced air heat will still distribute the humidity it produces through the house with no problem. It doesn't require the babysitting that freestanding humidifiers do, though- no daily tank refilling, no messing with nasty wicks, no fungus issues.
Basically the guts are the same as in a power flow-through humidifier, except you mount the 360 in an interior wall instead of on the furnace plenum. The fan draws air from one side of the wall and blows it through the wick and out the other side. Since you don't put any heat into it from the heating system, it needs a hot water supply to make a useful amount of evaporation take place. The output is probably still a bit less than what some other systems can do (with the hot air from the furnace plenum), but this is a good alternative when other systems aren't practical. Domestic hot water is good enough to steam up the bathroom after a shower, so it can still do a lot with good airflow going across a flow-through wick.
Hey, your having the same issue as I am!
New House, Carrier Infinity, wanted a bypass hum in attic but it would need freeze protection. Wanted to do that but it was to expensive. Anybody know if the 360 can go with the Infinity Fan Coil with an Infinity Control?