Have you seen the beginning of MTS 31? Eddie demonstrates a ninja choke counter to capoeira. If that's not your cup of tea (personally, I'm not sold on the ninja choke yet), at the 4:15 mark he talks about another counter whereby you fight the elbow in the armpit and regain your underhook on that side. Then you threaten to come on top by bridging into him, this should get him to react by swinging his leg back over, and now you have him in half guard.