How would santa claus be handled in ars magica? 
My thought is that he is a repentant fey. 8)
en.wikipedia.org/wiki/Santa_claus
Santa Claus is largely based on Saint Nicholas of Myra, who was known for his generosity to the poor. Thus, in most of mythic Europe, he would be aligned with the Divine Realm, giving gifts to the pious poor.
However...
In the Germanic tradition, children often left gifts of straw, sugar, or carrots in their boots near the chimney; Odin led a great hunt on the Germanic Yuletide, and Sleipner (his eight legged horse, who could leap great distances) would get hungry. Thusly, he would reward the kindness of the children with small gifts or candy. Thus, in the Rhine Tribunal, "Santa Claus" would be a creature of Fae realm.
However, as years roll, and traditions get muddled, It would probably be safe to say that Santa Claus is a fae who is aligned with the Divine, much like the church faeries. He is kept alive by the stories and dreams of man and child, and represents charity, kindness, and generosity, things most pious Christians strive for.
In truth, it's really up to the individual to decide.
Steve
This was already asked, (and given it's due?)