As an Chinese expat, I have indeed heard rumors about undercovers from Chinese three letters agencies in events organized by dissentients (like falungong) and I would try my best to avoid being involved.

However I hope the collateral damage to the innocent Chinese expats can be minimized. The reason of many of those to stay outside of China are concerns about the the ruling party and the desire to keep a distance from it. (Admittly also a higher quality of life, but no longer true in many cases.)