Trees can live in pots so sage is nothing. ;-)

Just make sure that you mulch it good - ie., treat it like you would in-ground but make sure the barrel has very good drainage and that you don't cheap out on the soil in it. Alot of container plants end up dying not because of the cold but because they were too wet and cold. If the barrel is out in the elements, the snow can give it some moisture. Otherwise consider giving a little drink on warm winter days when the air temps are above freezing and the soil isn't frozen.

I have 6 kinds of sage in all different sizes and they all live happily in pots. Of course, being in Cali, i have no idea about overwintering things.

I've had mixed success overwintering sage outdoors in smaller pots. In a barrel it should do fine, especially if you insulate the sides of the barrel for the winter with bubble wrap or a few layers of burlap, and put a few inches of mulch on the soil.

Sage can also be cut back pretty drasticly to keep it small. I cut mine in spring after blooming. Sage roots can take quite a bit of freeze. (Assuming we are talking about Salvia officinalis, culinary sage.) Even where I grow mine in the ground, I have it first in a clay pot, slightly elevated above the garden soil to aid drainage.