Rotating around a pivot point using a Quaternion?

Here is a basic implementation of RotateAround;

 static void RotateAround (Transform transform, Vector3 pivotPoint, Vector3 axis, float angle)
 {
     Quaternion rot = Quaternion.AngleAxis (angle, axis);
     transform.position = rot * (transform.position - pivotPoint) + pivotPoint;
     transform.rotation = rot * transform.rotation;
 }

The basic idea is to move the rotation's origin to the pivotPoint by taking the vector between it and the transform's position, then move back after - the rotation is then applied like normal. If you want to do the same with your own rotation, then you can just replace the call to AngleAxis;

 static void RotateAround (Transform transform, Vector3 pivotPoint, Quaternion rot)
 {
     transform.position = rot * (transform.position - pivotPoint) + pivotPoint;
     transform.rotation = rot * transform.rotation;
 }