pinky, I agree he was a tackled player, 'sitting' on the tackler.
I dont understand what you're getting when you say ........
Surely its whether or not a tackle (as defined) is effected, that precludes a maul being formed, rather than the exact location of his grasping???
If the BC is already bound to his own player, then the opposition player grasping him above the waist forms a maul and if he brings him to ground he has collapsed the maul. If he goes below the waist, and he can do thisas it is still open play, he can tackle the BC and tackle law then applies if he brings the BC to ground. If that happens, the tackled player is not allowed to hold on to the ball and get back to his feet.