If you target the arm, clasping the hands together won't matter. There's no way he can stop you from digging in with an arm and breaking his grip. You have your whole lower body attached to his neck, so you can move him closer or farther from your torso, and your whole upper body on the arm, hunting for some form of a kimura or americana grip. Just keep moving around, and your hands will find places to go to break the grip eventually. Don't forget that you can sub the arm either way or straight with that grip. Just separate the arm, and then take what he gives you.