Normally to apply a rotation stored in a Quaternion you would just multiply the quaternion by the Vector3: Quaternion.Euler(0f, 45f, 0f) * Vector3.forward. This is part of the Quaternion multiply operator.
I was thinking that it would be possible to add an implicit operator but you can only define them within the class. A helper method on the Vector3d class would work or a helper extension method of Quaternion would really help out when working with Quaternions with these classes.