I'm not sure what sort of ceremonial functions you expect this athame to perform, so it's had to give you a really solid answer.
I know that many people use an athame to draw the perimeter of a magic circle, and for that reason use something intimidating like a sword or a deadly knife. The idea is to create a warning that says something like "beware, the person in this circle can be dangerous." More ancient customs involve digging a trench with a dagger and spilling sacrificial blood into it, and authorities on oriental mysteries identify the tool as a blood-drawing weapon specifically. In light of all this, it would certainly not be out of line to use something associated with "violence," especially not game-hunting from which activity a stray arrowhead might have come, unless perhaps you are a pacifist vegetarian.
My only advice on the matter, which I thought it prudent to mention, is to thoroughly dry your willow branch beforehand. It may help to use a piece that is extra-long, bind the ends, and be exceedingly patient as willow is soft.
--------------------
|