In this video, Kino Macgregor demonstrates some useful techniques for getting close to the pose. Most important is moving the weight all the way onto the half-lotus knee and off of the opposite hip. (Don't clench or grip the lotus knee, just allow it to support the weight.) If you lean back (as is natural) you won't be able to get the tight package you need to get the arm around.
Also: since this pose is basically impossible without a lot of preparation, use an intermediate posture of hugging the knee while twisting the spine. Achieve an opening in the twist before obsessing over binding the arms. Though Kino doesn't mention it, sitting on a folded blanket of course helps with the crazier sitting poses.