YouKnowWho, that is a good question about bong sau. I think that it can be vulnerable to arm-lock if someone is still learning it and if they don't really have the transitions and flow that accompany it. I think it would be very difficult to lock someone who is good at it, a very low percentage technique at that point.

The arm position by design is one that momentarily 'locks' the wing-arm into a posture where the tendons are 'bound'/preloaded and it is a position that one leads an arm into to apply various chin na techniques such as the basic wrist grab or the small wrap hand counter. Or to straighten the arm into elbow/shoulder lock. Even knowing how to apply these locks well, it is nearly impossible to apply them on someone with a decent bong sao, because some of the basic chin na counters are exactly the way the arm is taught to automatically flow away from the lock anyway. Could flow into things such as tan sao (elbow down), lan sao, cutting elbow, etc. And in addition to the arm movement, the body can turn and pivot the angle of the hips and change the dynamics to neutralize the attempt. Same results against someone trying to catch a lock with a Jiu-Jitsu/BJJ background too; they won't hit the lock unless someone is either pretty unskilled or just falls asleep.

Anyway thats my perspective/experience. Interesting discussion.