I don't know much about Tai Chi myself.
Hopefully, the WTS won't come out with anything about Tai Chi, but I do think it's possible. Do they allow other types of martial arts for JWs? I'm not really up to date on things like this. However, the little I do know about it is that Tai Chi is based both on the practice of Qigong and the various martial arts....in fact, it is the "martial arts" segment of Qigong, which has principles similar to Yoga.
My daughter has been attending a Tai Chi class, and she has found it very helpful for both exercise and relaxation without being strenuous.