As much of a fan of the Taig as I am, there were a couple of aspects about the whole Z column that I never liked either. That being said, a properly trammed and torqued Z column should be able to take about anything the rest of the machine could ever reasonably throw at it. Rigidity has never really been a complaint I've heard as regards the Taigs. As little bench mills go, it is pretty stout.
Maybe you could simply drill and pin the connection after tramming if you are worried about it simply shifting, but if you really are running the mill hard enough to get the Taig actually flexing, bracing probably won't help - you are probably doing serious enough work that you really should be looking at full-size mill instead!