Is Behe's experiment (evolving the bacterial flagellum) plausible in the lab?
The experiment itself is technically feasible - but whether there is any point (as opposed to using the resources to perform a different experiment) is highly questionable. The problem is this: Behe's claim is that the mechanism is irreducibly complex, and that it could not have arisen by natural selection. This is theoretically falsifiable, but the experiment he proposed does not provide a good dichotomy - there is a very large chance that he is wrong but over the experimental term of 10,000 or 10,000,000 generations, the flagellum might not arise.

The absence of the flagellum in the experiment is the most likely scenario, and doesn't either support or falsify his claim. It took evolutionary time, something on the order of 1010+ generations, to arise the first time (according to the consensus claim); we could not expect similar complexity to arise for the same purpose under conditions of directed evolution. That's a terrible experimental design: huge resource input, huge likelihood of no result.

To be clear: it is technically feasible, but not a good experiment. He proposed it because he knew that nobody would do it (because it's a bad idea).