You wouldn't want the mesh collider to update with a skinned mesh even if it was possible, since it would be very slow. You can attach primitive colliders to bones instead.

I found a script to do this on the forum from user Darkrobyn. I believe this code reproduces the calculations that Unity is doing internally to deform the mesh.

It's working for me, but I currently have a fairly simple mesh and bone setup. I'm firing the forceUpdate occasionally (on mouse up events) which works great in my application.

Have a look here:

http://forum.unity3d.com/threads/14378-Raycast-without-colliders

The poster didn't include a couple of custom classes that you'll have to add. Something like this:

class CVertexWeight
{

    public int index;
    public Vector3 localPosition;
    public float weight;

    public CVertexWeight(int i, Vector3 p, float w)
    {

        index = i;
        localPosition = p;
        weight = w;
    }

}

class CWeightList
{

    public Transform transform;
    public ArrayList weights;
    public CWeightList()
    {
        weights = new ArrayList(); 
    }

}