Dododecapod, I hardly think that this idea is as outright deadly as you believe. First, the enema would not put the bowls under much more pressure than a normal enema. The point of it is not to build up enough pressure to force vomitting, but to fill the sub up enough that she may vomit, with a little encouragement. The intestinal bacteria are more durable than you make it out to be, considering everything that they endure over a lifetime, and the valves of the human digestive system are not absolute, as anyone who's ever suffered motion sickness can attest. There is some concern with the actual vomitting, but it should be an acceptable risk, provided the sub is under continuous supervision and has a workable safe signal.

That said, I never considered the possibility of infection from intestinal bacteria. That may be enough of a concern to put this already risky scene over the edge.